Celestia Forums

Hi to all the sky watchers

I am new to this hobby, and my telescope is a 16x50 binoculars but not for long.

I would like to know, the first point of Aries, the cross point of the equator
and the celestrial equator, what is that points coordinates on earth, the lat will be 0 dec. The RA is 00h00m00s at that point

I know that that point is moving ( it will move around in 26 000 years ) but all the stars RA was re-define in 2000, this is done every 50 years.

I want to calculate my home's hour angle.

If any one can help me, please send a email to me aerasmus@cadutoit.co.za

Regards

The "hour angle" associated with your home moves through all 24 hours of RA during each day as the Earth rotates -- if you're considering the point on the sky that's directly overhead.

You'll have to tell us what you're actully trying to find out.

Ok, let me try to again, the zenith of the sun at the equator, that should be the RA zero point, and in terms of the latitude and longitude on the earth there must be coordination points , as the equator is one of the lines that should be 0 deg. but there must a longitute ref. line - west or east of Greenwich - a pitty I can not draw a pitcure

As I see is, you have two circle's with a 23 deg angle. The point where they cross on the equator must be a "fixed" point, and that is a distance from Greenwich

AMErasmus wrote:Ok, let me try to again, the zenith of the sun at the equator, that should be the RA zero point, and in terms of the latitude and longitude on the earth there must be coordination points , as the equator is one of the lines that should be 0 deg. but there must a longitute ref. line - west or east of Greenwich - a pitty I can not draw a pitcure

As I see is, you have two circle's with a 23 deg angle. The point where they cross on the equator must be a "fixed" point, and that is a distance from Greenwich

The celestial equator is simply the projection of the earth's equator onto the (imaginary) celestial sphere. Both equators lie in the same plane, the earth's equatorial plane. The earth's orbit lies in a different plane, called the ecliptic plane. These two planes, the equatorial plane and the ecliptic plane, intersect. The direction of the line formed by the intersection of these two planes changes very slowly, due to precession of the earth's axis, which changes the orientation of the equatorial plane.

The sun always lies in the ecliptic plane, because it's at the center of the earth's orbit. When the sun is over the equator (which happens twice a year), it also lies in the equatorial plane, so at that point it lies on the line formed by the intersection of the two planes. The direction of this line determines the first point of Ares (aka the vernal equinox).

The RA of a celestial obect is measured from the vernal equinox, so when the sun is there its RA is zero. As the earth moves in its orbit around the sun during the year, the RA of the sun slowly changes. As the earth rotates during the day, the RA of the zenith point at a given location on earth changes continuously, as does the location of the point on the equator at which the vernal equinox is overhead.

Does that help answer your question?

- Hank