First point of Aries

[MartinReeve]

Hi Guys, I'm totally new to this, and am struggling to understand how to locate the First Point of Aries. It seems to me this ought to be a fixed point on the celestial sphere which will change position in the sky as the earth rotates. Is that right? Someone give me a hand with this while I still have my sanity!

[adamsp123]

The first point of Aries is the point where the earths equator..... oh here is a link that explains it better than I could LOL. http://www.glyphweb.com/esky/concepts/firstpointofaries.html

[Doc]

The first point of Aries is a place in the sky where the ecliptic and the Celestial equator cross, and when the Sun reaches this point, as it does once a year, the Vernal Equinox occurs. The first point of Aries also marks the Celestial Meridian, which is the zero-point for calculations of Right Ascension.

It is interesting to note that the "first point of Arties" is actually a bit misleading today. When the equinox was first observed, many thousands of years ago, this point actually did lie in the constellation of Aries. However, due to precession of the earth's pole, the first point of Aries crossed into the adjacent constellation of Pisces in about 70 BC. So it should actually be called the "first point of Pisces" since we now live in the "Age of Fishes".

The Point's journey is now about 75% complete through Pisces in another 600 years, it will pass into Aquarius.

[adamsp123]

Yea - just what Doc said I got keyboard fatigue!

[MartinReeve]

Thanks chaps, I think it might slowly be sinking in to my earth bound brain. But before I get too carried away, does this mean that the point will be over-head at midday at the vernal equinox (21 March?)

[kniclander]

I really struggle with this stuff. An excellent book is "Guide to the Celestial Sphere".

In answer to your question, I think the answer is no - the sun is only ever directly overhead between the tropics of cancer and capricorn. I think

[kniclander]

actually, it's called "The amateur astronomer's introduction to the celestial sphere" by William Millar

[MartinReeve]

agreed kniclander - I didn't mean over-head (I'm going to have to learn to think more accurately). Could I say then that the sun will be in line with the first point of aries at midday on 21 March? It's the point where the apparent path of the sun intersects with the celestial equator, right? So, I'm trying feebly to visualise where that point is in order to get my bearings - it is after all our fixed point of reference for RA measurements. Right? (please tell me I'm right!)

[barkis]

You would do well to establish a true north south line where you are locally positioned, the get yourself a Sidereal Clock which will tell you what star time it is continually, as it crosses your local meridian.

You can the consult an ephemeris which will tell you when any object will south at your exact location.

Ron.

[kniclander]

agreed kniclander - I didn't mean over-head (I'm going to have to learn to think more accurately). Could I say then that the sun will be in line with the first point of aries at midday on 21 March? It's the point where the apparent path of the sun intersects with the celestial equator, right? So, I'm trying feebly to visualise where that point is in order to get my bearings - it is after all our fixed point of reference for RA measurements. Right? (please tell me I'm right!)

you are right (I think...)