# Stars Celestial Coordinates always the same...?

Since we recently had an Astronomy topic… and I’m learning too…

Help me understand why a stars Celestial Coordinates are always the same… I don’t understand why as the earth spins ( and moves around the Sun ) the coordinate to dial in to see a specific star can always be the same…

Because celestial coordinates aren’t fixed to the Earth. It’s like asking how it is that a town is always at the same mile marker on the freeway, even though your car is zipping past the mile markers.

They’re not. First of all, stars have some relative velocity in space, which results in nearby stars having a measurable “proper motion”. More importantly, if you look up equatorial coordinates in a catalogue and want to know where to point your telescope, you must take into account that the Earth’s axis is rotating like a top due to axial precession, and there are other effects like “nutation” as well. In fact, the catalogue will specify that the coordinates are valid for a particular epoch, like J2000.0, and from that you will be able to take into account a whole bunch of effects to calculate the apparent position of the star in the sky.

That includes the effects of the Earth orbiting the sun which will result in parallax, aberration and such.

ETA: for precise astrometry there is a fixed celestial reference frame defined that is supposed to be fixed and non-rotating with respect to infinitely distant objects.

But in this Car / Mile Marker- Example; As you drove away and off down the road, and made turns and wound around curves in the road as you drove away, the DIRECTION back would have to look from your car to the mile marker - Town would be changing…

Since I am the one who asked the other question referred to by the OP, I have an interest in that. And I think I can contribute on this one.

There are actually several coordinate systems in use when observing the sky. One is the altitude/azimuth system, which uses two coordinates: Altitude, defined as the angle of the star above the horizon (horizon itself is 0°, zenith is 90°). And azimuth, defined as the direction on a compass - north is 0°, east is 90°, south is 180°, west is 270°, just as you’d use to designate the course of a ship or airplane. In this system, stars do not have fixed coordinates; they change all the time as the Earth rotates.

The other system is the celestial coordinate system with declination and right ascension, which is analogous to the latitude/longitude system on Earth: The celestial equator (the projection of the Earth’s equator into the sky) has a declination (analogous to latitude) of 0°; the celestial poles (the points in the sky the Earth’s axis of rotation points at) have a declination of 90° north or south; and the point where the Sun stands at the beginning of spring has a right ascension (analogous to longitude) of 0. Right ascension is measured in hours rather than degrees (with the full circle having 24 hours), but otherwise it’s the same logic as longitude on Earth. In this system, coordinates of stars are defined relative to the celestial poles and equator, and they are at fixed points in the sky, so the coordinates of the stars in this system never change, even though the azimuth and altitude which correspond to them at any moment do.

The so-called “fixed stars” may be fixed in the sky, but the celestial poles and equator definitely aren’t. For instance, right now α Ursae Minoris is pretty close to the north pole, but in 12000 years or so it will be Vega.

Yes, and so you’d never publish something giving a town’s location as “20 miles southeast of your car”. You give the car’s location in some frame of reference where the town is fixed (like highway mile markers or latitude and longitude), and then let the people in cars figure out, based on that and their own location, where they can find the town relative to their cars.

Likewise, you give a star’s position in celestial coordinates, right ascension and declination, and then let astronomers figure out based on that where to aim your telescopes.

This is true, but over human timescales, those motions are insignificant.

Depends what you mean by significant; I will merely point out that if you crack open a nautical almanac, which is designed for practical purposes, the tabulated positions may be regarded as constant over any period of several days, but are slowly but surely drifting if you look at them month to month. The given precision is 0.1 arc minute.

Thanks… THAT, especially that second paragraph, helps me understand why that works better than any book I’ve found.

Incidentally, there’s a reason why right ascension is measured in time units, not angle units. Observatories have special clocks called sidereal clocks, which run through a full “day” in a few minutes shorter than regular clocks. The idea is that, while a regular day is the time it takes the Sun to make a full cycle, the sidereal day is the time it takes a star to make the full cycle. And a star’s right ascension is the sidereal time at which it’ll be at the highest point in the sky, and hence easiest to observe.

So are there less than 24 hours of right ascension, seeing as a sidereal day is slightly shorter than the civil day? Or are the hours of right ascension, and by extension the hours of which a sidereal day is defined to have 24 of, a different length?

There are 24 hours of right ascension. You may think of sidereal time as the hour angle of the equinox. A sidereal day on Earth is shorter than a solar day by a few minutes, so a sidereal hour is shorter.

(Note: solar time is itself not exactly the same as uniform atomic time, the equinox itself is precessing, etc., when it comes time to be really precise)