# Right Ascension for epoch 2000 - physical location?

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I understand that Right Ascension is a longitude-like celestial coordinate that varies from 0-24hrs, taken from a reference point of the vernal equinox. More specifically, for star maps that are based on the epoch 2000.0, this specifically means when the vernal equinox occurred in the year 2000. Even more specifically, the vernal equinox is a specific time and date, and location of the exact point in time, when sunrise occurred, of when the day and night were exactly equal in length. Here are my questions:

1) If the Right Ascension is referenced from “Vernal Equinox” position “0hr”, in the year 2000, then why does nobody ever talk about the equivalent physical location? ie: if the right ascension (sunrise of the vernal equinox) occurred in the year 2000 in a particular city, or longitude, why doesn't anyone ever mention that. ie: (just an example) Right Ascension is always measured from longitude -80deg (close to Miami, Florida, USA), as this was a city the vernal equinox occurred in the year 2000?

2) If the star maps do not change much within a period of a few decades, (eg: take for example, the right ascension and declination of the sirius star is pretty much the same in 2019 as it was in the epoch 2000, or 1990 for that matter), then these RA and dec values should not change much within a decade or so. Ipso facto, we can deduce that the sunrise time for the vernal equinox should not change very much from year to year. ie: Since RA is measured from a reference based on the sunrise of the vernal equinox in the year 2000, then the sunrise of the vernal equinox in the year 2019 should not be very much different (assuming RA is the same today as it was in 2000). But it isn't!!! If you track the sunrise of the vernal equinox from year to year, it varies a lot! I am obviously missing something… it's driving me nuts.

Update below:

From Norton's 2000.0 star atlas, please see the following definitions:

Norton's 2000 Star Atlas Excerpt 1/3
Norton's 2000 Star Atlas Excerpt 2/3
Norton's 2000 Star Atlas Excerpt 3/3

In the last link, It says the 0 line of right ascension is equivalent to the Greenwich meridian on Earth. In a sense, this is the ultimate answer to my question #1, although it begs more details. In Astronomy, why is this rarely mentioned? If it was, star maps would be much easier to comprehend by anyone. (Ie: RA of any star is simply referenced from the Earth equivalent of longitude 0… ).

Does this also mean, that during the vernal equinox, the point in time where the sun crosses from the Southern Hemisphere to the Northern hemisphere, longitude 0 meridian on Earth is directly at that intersecting point too? So, as Mike G said, “sunrise" is irrelevant on the day of the vernal equinox. I understand this part. But the exact moment of the vernal equinox does matter… because from any point on Earth, the stars all move in the sky as time progresses, hour by hour. If you're going to base a map of where the stars are, it has to be referenced from one moment in time, and from once longitudinal reference (and one declination reference).

Now, bringing it all back to star maps: @Mike G: Would you say that simply, any star map based on J2000 epoch is basically where all the stars were on Jan 1, 2000 (at 12 noon greenwich time)? However, you also mentioned the J2000 epoch does NOT depend on the equinox. This is where I go fuzzy. From reading the reference links above, it seems like it does depend on the equinox. If the J2000 epoch star maps are indeed referenced from 12 noon, then in the year 2000, at the time of vernal equinox, the greenwich meridian intersection occurred at 12 noon?

Update 2
OK, I think I have some of my basic misconceptions cleared. RA 0hr simply cannot be measured relative to any point on Earth. It is measured against the first point of Pisces (It used to be Aries a long time ago). Specifically, for J2000 star maps, the RAs would be calculated based on relative positions from Pisces in the year 2000 around March 21 (vernal equinox). (Norton's Atlas -2nd link above- describes RA as around this date. I found no reference for January 1 -please correct me if this is not the case).

So to summarize what I understand, if you're only given an RA and declination for a particular star and you wanted to find it in the sky, you would need to innately know where Pisces is. From there, you can roughly work out approximately how many degrees of "longitude" away from Pisces your star is. (So, you would also need to know where due North and due South are). You would also have to know what latitude you are at on Earth, so you can work out roughly where your star's declination places it on it's meridian in the sky. For example, if you knew you lived at +40deg latitude on Earth, and you looked straight up at the zenith, you could estimate that the horizons are at about +90deg and -90deg away from that +40deg position. For there, you could interpolate where on the meridian (declination) your star lies.

Update 3
Thanks Mike G for the wiki reference to Celestial Coordinates. This shows that the star atlas J2000 is NOT referenced to the vernal equinox, but simply references to a snapshot in time of the stars on January 1, 2000, noon terrestrial time (or 2000 January 1, 11:58:55.816 UTC, which was the old greenwich mean time). The RAs and Decs in the atlas are referenced to Pisces at that moment in time.

Fortunately things aren't as entangled as you thought. The J2000 epoch does not depend on the equinox; it is simply 12:00 TT on January 1, 2000. The time of vernal equinox is unrelated to sunrise anywhere; it is just when the Sun appears to cross the celestial equator northward as seen from Earth.

The vernal equinox point ♈ is located at the intersection of the equator and the ecliptic. Precession changes the equatorial plane over time, shifting its intersection with the ecliptic plane by ~50" per year or ~1.4° per century. The J2000 equinox is where ♈ was at the J2000 epoch.

The variation in equinox times is an artifact of approximating the tropical year's odd quarter day with a whole day every four years.

The Greenwich meridian and the 0h hour circle have analogous roles in their respective coordinate systems, but one rotates with the Earth and the other doesn't. They align at 0:00 Greenwich sidereal time each sidereal day. Local sidereal time corresponds to the right ascension aligned with the local meridian at that moment.

As the Sun appears to move ~1 degree per day along the ecliptic, sidereal time runs faster than solar time by ~4 minutes per day or 1 day per year. Solar and sidereal times match when the Sun is at the autumnal equinox ♎ (RA = 12h) and differ by 12 hours at the vernal equinox.

The difference between local sidereal time and a star's right ascension is the star's hour angle east or west of the local meridian. Using this, any star of known identity and right ascension can serve as a reference for locating other stars by coordinates. Equatorial setting circles rely on this.

There are several meanings to "vernal equinox", and I think you have not picked the right sense here.

The vernal equinox is here taken to be a point on the celestial sphere. It is the point where the projection of the Earth's equator to the celestial sphere and the projection of the Earth's orbit intersect. (There are of course two points, the other is the autumnal equinox). This is also called the "point of Aries" (despite the fact that it is currently located in Picies) This point always exists and only moves very slowly. It is a point in the sky, not a point on the Earth.

The other meaning is the time when the Earth moves from the the portion of it's orbit that is in the south, to the part when it is in the North. This occurs around the 21st of March each year. (The exact date varies, due more to the calendar than to irregularities in the orbit). There is no place on Earth that is at the vernal equinox. In this sense it is a time, not a place.

Right ascension is measured from the Point of Aries, and in particular the location of the Point of Aries at the start of January of the year 2000 (hence 2000.0) The point of Aries moves slowly, but quickly enough for there to be a need to update stellar coordinates every 50 years.

The right ascension is not measured from the sunrise at the vernal equinox in 2000. It is measured from the Point of Aries in January 2000.0

In the Norton sky atlas is says "the zero line of right ascension is the equivalent of the Greenwich meridian". This is correct in the sense that they play the same role in there respective coordinate systems. It is not the case that there is any other relationship between the two lines. The zero line of right ascension is not the projection onto the sky of the Greenwich meridian.

I'm not sure what you mean by "rarely mentioned". It is implict in any map of the celestial sphere, which would show lines of constant right ascension as great circles from one pole to the other. This point is not deep or complex.

It does not mean that longitude 0 is at any particular point at the moment of equinox. The exact time of equinox can occur at any point in the day-night cycle. At that time, the Greenwich meridian could be pointing in any direction, relative to the stars or to the sun.

The stars do move in the sky, but they do not move (much) on the Celestial sphere. The definition of the celestial sphere means that the stars are almost fixed and I can talk about the coordinates of a star. Even as the stars appear to travel from East to West, their celestial coordinates remain unchanged.

Stars only appear to move slowly due to the precession of the equinoxes, nutation, the actual movement of the star in space and the slight wobble of parallax. Distant stars and galaxies don't have any measurable actual motion or parallax.

So when I give coordinates of a star position, I need to define my frame of reference. One way to do that is use the actual position of the Point of Aries at the current time. However the very slow precession means that even distant star's coordintes will change over the years.

Alternatively I can use the position of the Point of Aries on Jan 1 2000. This means that distant stars will be completely fixed in this coordinate system.

To put it another way: The J2000.0 system has a point of aries (or vernal equinox) that is fixed in relative to distant stars.

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## I | The Nearest Stars, Brown Dwarfs, and White Dwarfs

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## J | The Brightest Twenty Stars

Note: These are the stars that appear the brightest visually, as seen from our vantage point on Earth. They are not necessarily the stars that are intrinsically the most luminous.

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## Talk:Epoch (astronomy)

Shouldn't be this defined in Terrestrial Time?

Yes, so why not change it now? Nike 06:53, 17 Dec 2004 (UTC)

Is it known how long the current epoch will be adhered to? Do we know what will come next? Are these questions relevant and if not, why not? Can this be clarified in the article? (As may be apparent, I don't have a clue.) -- Cimon avaro on a pogostick. 11:58, 14 January 2006 (UTC)

Usually it's every 50 years, although I'm not certain that this is always the case. The article refers to B1875.0, which is only 25 years before B1900.0. --Nike 03:45, 16 January 2006 (UTC)

Should there be a reference to Epoch/Unix time here - 1453330082 66.194.64.130 (talk) 22:48, 20 January 2016 (UTC)

What's with all the #s? I've never seen this usage anywhere. #Julian epoch is just bizarre. Also, why are there links to articles which no longer exist?

Although the article only mentions one Julian epoch, I have also seen J1900 and J1950. --Nike 08:51, 3 August 2006 (UTC)

I get it, those are supposed to be links to sections within the article. You need to use a piped link. That would look like this: [[#J2000.0|J2000.0]] --Nike 09:00, 3 August 2006 (UTC)

What are Besselian vs. Julian epochs? How do they differ from each other aside from what letter is prefixed to their number? --Haruo 10:03, 6 October 2006 (UTC)

What about the article is unclear in explaining this? --Nike 11:38, 6 October 2006 (UTC)

What I want to know is why the phrase "Since the right ascension and declination of stars are constantly changing due to precession. " is repeated verbatim in every section. BIEB!! 13:07, 25 October 2006 (UTC)

My guess is that this article is actually a series of stubs that were stitched together in the hope that someone would clear the result up into some coherent whole sadly, nobody seems to have done so yet, and the result is just a mess. - IMSoP 01:03, 5 November 2006 (UTC)

This page may be mis-titled completely. When speaking of terms such as 'B1950.0' and 'J2000.0', especially in the context of celestial coordinates and precession, you are speaking of EQUINOX, not EPOCH. For example, the EPOCH of observation on a star may be 1991.25, but the coordinates given for its posistion may be specified in EQUINOX J2000.0 (The Hipparcos and Tycho stellar catalogs are a good example of this). Radec 08:15, 21 January 2007 (UTC)

I was directed to Epoch by the Comet Infobox on the Comet McNaught article, which states "Epoch: 2454113.2961 (January 20, 2007)". It was not immediately apparent from this article that the stated epoch was in fact a Julian Day, and that I should refer to that article for an explanation of the number 2454113.2691. If a subject expert is revising this article, it would be helpful if they would consider this usage. PaulKishimoto 17:57, 21 January 2007 (UTC)

Julian year 2000 began on 2000 January 1 at exactly 12:00 TT.

Does that imply according to the Julian calendar? —Preceding unsigned comment added by Trigamma (talk • contribs) 22:58, 4 January 2008 (UTC)

That and other references within the article to Julian epochs are poorly worded. For J2000.0, 2000 January 1 (at noon) is in the Gregorian calendar. Other Julian epochs differ from this epoch in Julian years of 365.25 days each. Hence the Hipparchus epoch of J1991.25 is 8.75 Julian years before J2000.0. This requires significant rewording of the article. — Joe Kress (talk) 08:17, 5 January 2008 (UTC)

Why is noon/midday written as 12h rather than 12:00 as in ISO 8601? – Kaihsu (talk) 05:58, 23 April 2014 (UTC)

I don't know why the editors who wrote this article chose to write "12h". The English Wikipedia has not adopted ISO 8601 to express time of day. Thus, if one just writes "12:00", it is unclear if one is referring to noon or midnight, because some articles use the 12 hour clock and others use the 24 hour clock. This article does not state whether it uses a 12 or 24 hour clock, so "12:00" is ambiguous. In the case of the 12 hour clock, MOS:TIME instructs "Use noon and midnight rather than 12 pm and 12 am ". So if you don't like 12h, the simplest solution is to replace it with "noon". Jc3s5h (talk) 15:46, 23 April 2014 (UTC) In astronomy, the time (and Right Ascension) is usually written as 00h 00m 00.00s. So Noon is written in full as 12h 00m 00s, 11:27 pm is 23h 27m 00s, and Midnight is 00h 00m 00s. (As a side note, Declination is written as 00d 00m 00s.) Dsgd47 (talk) 21:46, 24 September 2015 (UTC) As far as I know astronomy does not have one single authoritative source that every single astronomer will acknowledge as the authority on time notation. In some fields, midnight can also be 24:00. Thus, when using the word "midnight", one must specify whether it is the midnight at the beginning of a certain date, or the end of a certain date. Jc3s5h (talk) 10:19, 25 September 2015 (UTC)

Since the date format in the article is not consistent, I will follow MOS:DATEVAR and use the format used when a date was first added to the article, which is month, day, year. Jc3s5h (talk) 14:28, 6 August 2018 (UTC)

I agree the format for dates is inconsistent. However, being an article about astronomy, there is an internationally recognized standard for epochs mandated by the International Astronomical Union (see for example https://www.iau.org/static/publications/stylemanual1989.pdf, page S29) that shall be followed. Marco.bs (talk) 10:54, 7 August 2018 (UTC)

Style manuals for astronomical journals apply to the journals that issued the manual, not Wikipedia. Wikipedia has its own style manual as well as a manual for dates and numbers and a guideline for citations. I think it's highly unlikely we would completely adopt the manual Marco.bs mentioned, or another like it, such as the author instructions for the American Astronomical Society. For example, few Wikipedia editors would want to use a 29 year-old citation style from IAU rather than Wikipedia's citation templates. But, in the past, isolated recommendations from astronomy journal style manuals have been adopted, such as the symbol for astronomical unit, au. Currently the year month day date format (e.g. 2018 August 7 or 2018 Aug 7) suggested by IAU and AAS conflicts with the acceptable formats listed at WP:MOSNUM. If this style of date were used in citation templates, the article would be disfigured with red warnings because the date format would be considered improper. If you feel the year month day format should be allowed in astronomy articles, please take it up at WT:MOSNUM. Jc3s5h (talk) 11:51, 7 August 2018 (UTC)

I assume the following is a typo? 1950?

"Julian years, e.g., J2000.0 for January 1.5, 1950, TT"

Also, dates in the form "January 0.9235, 1950 TT" are beyond me. I consulted TT (terrestrial time) to learn what that phrase means and find no examples or templates there to explain such a date. "Julian Day" explains its own use of fractional days, but I haven't found anything to confirm any of my guesses as to how to specify such a day with a month.

Yes, "Julian years, e.g., J2000.0 for January 1.5, 1950, TT" is a typo and I fixed it. I am not aware of a widely accepted standard for how to specify a fractional day with a month name. I see that the Explanatory Supplement to the Astronomical Almanac, 3rd ed., in the glossary entry for J2000.0 uses 2000 January 1.5 TT, but our WP:MOSNUM does not endorse this notation. Jc3s5h (talk) 21:00, 1 October 2018 (UTC)

January 0.9235, 1950 TT = December 31.9235, 1949 TT = December 31, 1949 22:09:50.4 TT. (Historically, the "January 0" concept arose because astronomers start the day 12 hours before non-astronomers do. Astronomers didn't want the year number to decrement "unnecessarily".)

## Right Ascension for epoch 2000 - physical location? - Astronomy

Telescope Exercise for Astronomy 250 Worth 10 points Due March 30, 2001

Finding Examples of Stellar Evolution with the 21-inch Telescope.

Posted outside Prof. Rieke's office (Rm 262), you will find sign-up sheets for observing on the nights of March 23,24 and 25. Sign up for one night and a one backup night in case of bad weather. You may find it convenient to review Lecture 1 which discussed astronomical coordinates.

The examples of stellar evolution that you will observe are planetary nebulae and globular clusters whose stars provide nice tests of how stellar works. Recall that a planetary nebula results from a red giant blowing off its outer layers which form the nebula. The core of the red giant is visible at the center of the nebular as a white dwarf stars.

You need to find in the library or on the Web coordinates for two planetary nebulae and two globular clusters which are visible at this time of the year and which are visible from Tucson. How can you figure out what is visible from Tucson now? Any object which has a declination>-15 ° can be observed with the 21-inch (if we were outside of the city and away from buildings, we could observe to closer to the horizon limit which is set by our latitude (32 ° ) and can be compute from -90 ° +latitude = most southerly declination that rises above the horizon). In practice, the 21-inch can point to about 15 ° above the horizon. We can figure out what range of right ascensions are visible at night in late March from the definition of right ascension. Recall that 0 hr right ascension = position of sunrise on the first day of spring = March 21. These mean that at sunrise a position with a right ascension 90 ° different will be overhead on the meridian at sunrise. Right ascension increase towards the east so the location with have a right ascension which is 90 ° /15 ° /hr = 6 hr less or 18 hrs. We would like to observe before midnight rather than at sunrise so we want to observe objects whose right ascension are even smaller - 6 to 10 hrs less so we can observe from

8pm to midnight. This means that you want to find sources with right ascensions in the range of 7 to 14 hrs and declinations greater than -15 ° .

Two good catalogs to use in selecting objects are the Messier Catalog (see http://www.seds.org/messier/ ) and the NGC or New General Catalog. Another possible source of information is the "Observer's Handbook" printed each year by the Royal Astronomical Society of Canada. When you select your sources, note down the names and coordinates for your objects. You also need to look at what epoch the coordinates refer to -- precession changes right ascension and declination for a source continuously. If no epoch is stated (you should seem something like RA(1950) meaning the right ascension refers to epoch 1950), it is likely that the catalog is assuming epoch 2000 but you might want to find the same object listed in another catalog that does give the epoch. Because you will not be observing at exactly the epoch for which the coordinates are given, you will need to compute the precession between the catalog epoch and the night of your observation. Here's a short recipe of how to do this:

For example: Compute the precession for the galaxy M31 which lies at 00h 42m 44.32s and +41 ° 16' 08.5" from 2000.0 to Sept 1, 2009.

To finish the calculation, you would add these differences to the original coordinates to get the position for Sept 1, 2009/

Names and catalog positions for 2 planetary nebulae and 2 globular clusters.

Coordinates for all four objects precessed to the day on which you observed.

The coordinates reported by the 21-inch when the object was actually centered in the eyepiece.

A brief written description or sketch of the appearance of the planetary nebulae and of the globular clusters.

## The Astrophysics Spectator

Astronomy has it own units of measure. They are the natural units for conducting scientific research from Earth-based observatories, and they persist despite technological developments that make them superfluous. The values of the standard units of measure are given in the table below in terms of cgs units (centimeters, grams, and seconds). The values are from the compilation of constants by Yoder. 1

 Astronomical Unit (AU) 1.495978706×10 13 cm Parsec 3.085677580×10 18 cm 3.262 ly (Julian) Mean Solar Day (d) 86400 s Mean Sidereal Day 86164.09054 s Tropical Year 365.2421897 d Sidereal Year 365.25636 d

The Astronomical Unit (AU) is the mean radius of Earth's orbit around the Sun. The AU is a natural unit because the relative distances between the planets is easily derived by applying Euclidean geometry to the observations of planetary motions. The expression of the AU in metric units, however, requires additional physics, such as knowledge of the speed of light. The value of the AU in metric units is therefore a modern result.

The parsec is the standard unit of length for expressing the distance to a star. This word is derived from &ldquoparallax arc-second&rdquo, which shows the unit's origin. The distances to the closest stars are determined by measuring how they move relative to very distant stars over the course of a year. As Earth orbits the Sun, the nearby stars appear to move relative to the distant stars. By measuring the angle that a star moves for a baseline of 1 AU, one can derive a distance by taking the inverse of the angle. If the angle is measured in units of arc-second, then the distance derived from this inverse is in units of parsec. A star at one parsec distance would move 1 arc-second in angle on the sky when Earth moves 1 AU perpendicular to the line of sight to the star. Over the course of a year, a 1 parsec star moves by 2 arc-seconds across the sky, because the baseline is the diameter of Earth's orbit, which is 2 AU. That said, understand that no star is this close to the Solar System.

The second value for the length of the parsec is expressed in Julian light years (the distance light travels in a Julian year of 365.25 days). Astronomers never use light years in their research&mdashthe light year is simply a tool for grasping the immense distances between the stars.

Time is a complicated topic in astronomy because some of the primary time standards are defined in ways that make them vary. Earth's wobble, the precession of Earth's rotation axis, and the slowing of Earth's rotation from tidal interactions with the Sun and Moon make the day and the year vary. There is also the issue of how one determines the starting point of a day or a year: relative to a fixed point in the sky, or relative to a point that moves over time.

The physicist's measure of time is the second, which is defined in terms of the transition between two atomic states of cesium 133. The mean solar day, which is approximately the time for a complete rotation of Earth relative to the Sun, is defined by fiat as 24 hours, with hours and minutes defined in the standard way. The standard that is broadcast by time stations differs slightly from the mean solar day this standard is Coordinated Universal Time (UTC), or more commonly Greenwich Mean Time (GMT). UTC is kept by atomic clocks. Usually the UTC day is the mean solar day, but occasionally a leap second is added to correct for the drift caused by the slowing of Earth's rotation. UTC midnight is always within 0.9 seconds of true midnight. The time of a complete rotation of Earth relative to a point on the sky is called a sidereal day this is measured relative to the first point of Aries (the vernal equinox), 2 which is a point on the sky that slowly moves as the Earth's rotation axis precesses. The mean value of the sidereal day is shorter than the Julian day by 3 minutes 55.9 seconds. The tropical year is the year that the Gregorian calendar is based on, and it is measured relative to the first point of Aries. The sidereal year is measured relative to a fixed point in space quasars are used as the reference points.

Positions on the sky are measured in terms of right ascension (RA or &alpha) and declination (dec. or &delta). These correspond to longitude and latitude on Earth. The declination is measurd in degrees relative to the celestial equator, which is the projection of Earth's equator onto the sky. The declination of the equator is 0°, the declination of the north pole is 90°, and the declination of the south pole is -90°. The right ascension is defined in units of time, with 0 hour at the first point of Aries, and the value of the right ascension at the zenith increasing as time passes. A full circle of the equator corresponds to 24 hours. Right ascension was defined to make finding objects with a telescope easier: the right ascension at the zenith changes by one hour in one hour of sidereal time.

Because the precession of Earth's rotation axis causes the first point of Aries to moves along the equator over time, the right ascension and declination of the stars change with time. To counteract this, astronomers traditionally express the positions of stars for the coordinate system of a particular date. Currently current standard is Epoch 2000, or the coordinate system for January 1, 2000. Epoch 1950 was used previously.

1 Yoder, Charles F. &ldquoAstrometric and Geodetic Properties of Earth and the Solar System.&rdquo In Global Earth Physics: A Handbook of Physical Constants edited by T.J. Ahrens, 1&ndash31. AGU Reference Shelf, No. 1. Washington: American Geophysical Union, 1995.

2 The equinoxes are the two points in the sky where the celestial equator and the ecliptic (the path in the sky that the Sun appears to travel along) cross. The sun crosses the first point of Aries, or the vernal equinox, at the end of spring, and it crosses the first point of Libra, or the autumnal equinox, at the end of fall. While at one time long ago the equinoxes were in the constellations Aries and Libra, they are now in the constellations of Pisces and Virgo.

The first edition of the Pocket Sky Atlas was based on Wil Tirion and Roger Sinnott's Sky Atlas 2000.0. Stars are plotted at their Epoch J2000.0 positions, where they appeared in the sky on 1 January 2000. Is the second edition also plotted at the same epoch?

Has the IAU made a decision when we will start using Epoch J2050.0, or some other epoch?

I have a collection of sky atlases, most bigger and more detailed than the Sky and Telescope Pocket Sky Atlas, but the Pocket Sky Atlas gets more use than all the others combined. I keep it close at hand when I'm reading about the sky, so I can quickly find what I'm reading about. The pocket atlas and a red flashlight always go with me when I'm out skywatching. The charts are organized so that it's easy to see what's visible at any given time from north to south with minimal page flipping.

I have a copy of the jumbo edition, but it stays on the shelf. The pocket atlas is the perfect size to use as both a quick desk reference and a handy field guide under the stars.

Roger W. Sinnott Post Author

Thanks for your comments, Anthony. You're right, these 2nd editions continue to use the equinox 2000.0 reference frame. This is so the coordinate grid will be fully consistent with the positions announced for newly discovered comets, novae, and other objects described in Sky & Telescope and other modern magazines and books. Also, most GoTo telescopes are designed to accept 2000.0 positions. (If an atlas were plotted for some other equinox, you'd have to correct for precession every time you went to plot something among the stars -- a huge nuisance.)

Eventually, I'm sure astronomers will switch over to 2050.0, and then it will make sense to issue new atlases accordingly. On the other hand, I've heard rumors some scientists would like to continue using 2000.0 forever! (Personally, I hope that doesn't happen. It would be like the terrible idea to abolish leap seconds.)

Thanks Roger. I appreciate knowing that 2000.0 coordinates will remain the standard for the foreseeable future. But it will make sense to update all the charts and computers to account for precession at some time in the future. Reality can be inconvenient, but ignoring reality causes deeper problems in the long run. The analogy to leap seconds is apt.

I don't understand the difference between epoch and equinox, but as a humble skywatcher I probably don't need to. If I can get within a binocular or finder scope field of what I'm looking for, that's good enough for me.

Roger W. Sinnott Post Author

"Equinox 2000.0" means the coordinate grid has its equinox (0h R.A., 0 deg Dec.) at the point along the ecliptic where it was at the start of the year 2000. The grid system and equinox precess, relative to the stars as a whole, by about 1.4 degrees per century along the ecliptic. This is the most obvious change over time.

"Epoch 2000.0" means that individual stars with a sizable proper motion are shown they were in year 2000 with respect to the bulk of other stars (as well as to the equinox-2000.0 grid). The fastest-moving star is Barnard's Star, marked by a "+" on chart 65 because it is fainter than the atlas's magnitude cutoff. In 20 years, this star has moved 0.28 mm (about 1/100 inch) northward on the regular PSA chart, or 0.36 mm on the Jumbo charts.

To get overly pedantic, the grid of these atlases shows right ascension and declination for "equinox, equator, and epoch 2000.0."

Thanks for the explanation. I wasn't making the connection between "equinox" and "first point of Aries." Now it makes sense. And, of course, that's where the term "precession of the equinoxes" comes from.
I appreciate you taking the time to explain both the concepts and their practical importance.

My GoTo star atlas. I even use the jumbo edition most of the time in the field. Easy to read and just the right amount of fine detail.

Roger W. Sinnott Post Author

Hi. How do these compare to the SkEye Pro app for Android?

Roger W. Sinnott Post Author

Sorry, I don't have an Android. Maybe someone else can comment. /Roger

There is one critical piece of information missing -- at least buried out of my sight! -- that I need in order to choose between the regular Pocket Sky Atlas and the Jumbo edition: how big are they?! Inches, centimeters -- either will do.

Roger W. Sinnott Post Author

Harold: Gosh, I'll try to get that included in the descriptions! Thanks for pointing out this omission.

The regular Pocket Sky Atlas has 6.5-by-9-inch covers and 6-by-9-inch chart pages. Chart scale is about 48mm per 10 degrees.

The Jumbo version has 9-by-12-inch covers and 8.3-by-11.8-inch chart pages. Chart scale is about 63mm per 10 degrees.

August 17, 2020 at 11:45 pm

The classic PE is my standard starting point (tho I endorse Interstellerum for fine detail). With my farsighted vision I've thought for a while about the Jumbo. The carbon star tabulation is a great idea and clinched the order for me! I look forward to receiving the JE when circumstances permit.

## Right Ascension for epoch 2000 - physical location? - Astronomy

Date and Time: Enter the JD of the observation with as much precision as possible in order to correct for Earth's orbital motion and rotation.

Right Ascension: Enter the target star right ascension on the date of observation for Epoch 2000. The entry may be in sexigesimal hh.mm.ss.sss or decimal degrees ddd.dd. Do not enter decimal hours.

Declination: Enter the declination of the target start on the date of observation for Epoch 2000. Use decimal degrees dd.dd or sexigesimal dd.mm.ss.sss.

Proper motion (PM) in RA and Dec given in milliarcseconds (mas) per year.

Longitude in degrees. Use + for east of the meridian. Mt. Kent in Australia is +151.855640. Minerva North and TRES on Mt. Hopkins are at -110.9520. Moore Observatory is -85.528475.

Latitude in degrees.Mt. Kent in Australia is -27.798217. Minerva North and TRES on Mt. Hopkins are at +31.6751. Moore Observatory is +38.344792 .

Elevation of the observation site in meters. Use 682 m for Mt. Kent, for 2607 m for Mt. Hopkins, and 229 m for Moore.

Redshift z: For highest accuracy, include z = (&lambdaobserved/&lambdaemitted - 1). Use 0.0 otherwise.

Submit: Sends the request to our server where we confirm the request validity, and return the velocity correction (m/s) to be added to your velocity in order to have a barycentric radial velocity.

## Pixel-Based KOI Vetting Statistics

Planetary transit false positives are commonly caused by light curve contamination from an eclipsing binary falling partially within the target aperture (i.e., the pixels used to collect and sum target flux). Two pixel analysis methods are used to identify such eclipsing binaries for unsaturated target stars: flux-weighted centroiding, which measures how the center of light in the collected pixels changes during a transit, and PRF-fit difference images, which localize the source of the transit signal. Both methods provide an estimate of the location of the source of the transit signal. When that source location is offset from the target star by more than 3-&sigma, it is likely the transit signal is due to a background source (note the caveats due to crowding described below). These analysis techniques use pixel-level data, available in the Target Pixel Files (TPFs). The resulting position measurements are compared with the Kepler Input Catalog (KIC) (Brown et al. 2011). Details on these centroid methods are found in Bryson et. al. 2013.

When the target star is saturated (Kepler magnitude larger than about 11.5) the centroid results given in this section are invalid. In this case manual inspection of the data validation reports can identify well-offset background binaries&mdashsee Section 5 of Bryson et. al. 2013 for details.

In flux-weighted centroid analysis, when more than one source is present within a pixel aperture, either fully or partially, then the combined center of light within the collected pixels will occur between the locations of the sources. When the flux from either the target or one of the nearby contaminants varies in a transit or eclipse, then the combined center of light within the aperture will move across the focal plane. This motion is called a centroid shift. The location of the varying source can often be inferred from the centroid shift. The size and direction of the centroid shift is measured using the flux-weighted (FW) mean, (e.g., the first moment of the pixel data). This mean is computed with every flux measurement (30-minute long cadence), creating a time series of flux-weighted means. The centroid shift is measured by comparing portions of the flux-weighted mean time series that are Out-Of-Transit (OOT) with portions that are In-Transit (IT). The flux-weighted shift of the IT mean from the OOT mean is given as Right Ascension and Declination shifts. The offset of the transiting source object from the OOT flux-weighted mean is computed by taking the product of the FW shift and the factor [1 - 1 / (fractional transit depth)]. The Right Ascension, &alpha (J2000), and Declination, &delta (J2000), of the transiting object calculated in this way are reported in the table. The &alpha and &delta offsets of the resulting source location from the KIC target star position are also reported. The uncertainties and significance of the FW shifts and offsets are provided but do not reflect systematics caused by crowding. The flux-weighted method can be very accurate when the target star is well isolated and the transit source is located (well) within the flux aperture associated with the target star.

The PRF-fit difference image method uses three images: 1) an average of Out-Of-Transit (OOT) Target Pixel File images from data that were obtained near but not during transit events, 2) an average of In-Transit (IT) image Target Pixel File images that were collected during transit events, and 3) a Difference Image (DIFF) that is the difference between the Out-Of-Transit and In-Transit average images. The difference image provides an image of the transit source (neglecting variability of field stars). The Pixel Response Function (PRF) is a convolution of the Kepler Point Spread Function model with a model of typical spacecraft pointing jitter, providing a system point spread function (Bryson et al. 2010). The PRF is fit separately to the OOT and DIFF images, providing a measured location of the target star (fit to the OOT image) and a measured location of the transit source (fit to the DIFF image). The offset of the transit source location from the target star is given in the table as Right Ascension and Declination offsets (&Delta&alpha,&Delta&delta) as well as magnitude (sky offset &Delta&theta).

PRF offsets can only be computed on a per-quarter basis. The single quarter (SQ) PRF offsets are combined by a weighted mean.

The target position measured by the PRF fit to the OOT images is vulnerable to crowding. Therefore an alternative PRF offset of the transit source (measured by the PRF fit to the DIFF image) from the KIC position of the target star is provided. Both the flux-weighted and PRF-fit methods will have systematic errors due to crowding when other stars appear in the aperture's pixels, though these error are smaller for the PRF-fit method compared to the flux-weighted method.