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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 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.

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  • $\begingroup$ I think this question could benefit from a migration to Astronomy SE $\endgroup$ Oct 18, 2019 at 10:34
  • $\begingroup$ @AlexRobinson Astronomy & Astrophysics questions are perfectly on topic on this site, so it would not benefit migrating away. $\endgroup$
    – Kyle Kanos
    Oct 18, 2019 at 11:20
  • $\begingroup$ Even though this question can fall under the umbrella of physics, it is probably more of a fit to Astronomy SE... I'll post the questions there. Thanks to all of the responses so far from David Hammen and garyp. $\endgroup$
    – WillK
    Oct 19, 2019 at 4:09

2 Answers 2

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The vernal or March equinox is both a point in time and a direction in space. The point in time occurs when the Sun as seen from the Earth appears to cross the Earth's equatorial plane, going northward. The direction in space is the direction from the Earth to the Sun at this point in time, with respect to the background of the "fixed" stars.

This direction in space changes from one year to the next due to the precession and nutation of the Earth's rotational axis. These changes are well understood and well modeled mathematically. The mathematical models can be used to interpolate where that direction would be at any point in time rather than just at the March equinox. Another way of looking at it is that the equinox of date is the intersection of the Earth's equatorial plane and the ecliptic plane at the date in question.

Equatorial coordinates in the J2000 system use the mean equinox of date at the J2000 epoch, i.e., noon Terrestrial Time on 1 January 2000.

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  • $\begingroup$ If the vernal equinox is both a time a space, and for the epoch 2000 it is for the specific year 2000, why isn't it commonly referred to a specific location (longitude) on Earth? (Since many star maps are referenced to the specific epoch 2000). $\endgroup$
    – WillK
    Oct 19, 2019 at 3:56
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Neither the direction in space nor the timing of the vernal equinox has anything at all to do with the rotation of the Earth. See the answer by @DavidHammen for a good, concise explanation.

So concerning your second question: the time that the Sun crosses the equator has nothing to do with the rotation of the Earth, so there is no correlation from year to year between the times of sunrise on the day that the vernal equinox occurs.

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  • $\begingroup$ true, but if star maps don't change much year to year, and their corresponding RAs do no change much, why does the sunrise time at the vernal equinox change a lot year by year? (ie: the star maps printed for epoch 2000 is commonly used for decades before and after the year 2000) $\endgroup$
    – WillK
    Oct 19, 2019 at 3:59
  • $\begingroup$ The time of sunrise is determined by the rotation of the Earth. The time of the vernal equinox does not. There is no reason whatsoever that that sunrise should occur at the same time every year. $\endgroup$
    – garyp
    Oct 19, 2019 at 22:33

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