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.
From Norton’s 2000.0 star atlas, please see the following definitions:
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?
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.
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.