# Thread: Right ascension and declination - vs lat/long

1. ## Right ascension and declination - vs lat/long

I'm attempting to understand the difference between the terms.

Latitude and longitude, as I understand it, refer to an object's location on the Earth's surface (a sphere in space, technically) Longitude is measured going west OR east from the prime meridian, with negative marking west and positive marking east.

Right ascension is measured going EAST from the prime meridian (er, or vernal equinox technically?) Why only the one direction? I'm assuming it has something to do with the rotation of the earth.

Also curious as to why latitude marking north/south is usually given first and longitude marking east/west is second when RA marking east/west is first.
Last edited by Lobster4221; 12-17-2012 at 03:24 PM.

2. ## Re: Right ascension and declination - vs lat/long

Right ascension is composed of lines which we projected out onto the celestial sphere which do coordinate with the rotational rate of the earth, 15 degrees for every hour of RA up to 24 hours RA (close to earth's rotation rate).

3. ## Re: Right ascension and declination - vs lat/long

Here are to web sites that might be of help to you.
Basics of Positional Astronomy
Celestial Coordinate System

4. ## The Following User Says Thank You to olddogg60 For This Useful Post:

Lobster4221 (12-17-2012)

5. ## Re: Right ascension and declination - vs lat/long

The sequence is just convention, no particular reason for it. DEC is North or South of the Celestial Equator the same as Latitude is North or South Earth's Equator. North being plus and South being negative.
Clear Skies

6. ## Re: Right ascension and declination - vs lat/long

It's a little clearer now. Is hour angle not used that frequently in astronomy then? I've got start charts that have labels up to 22 hrs right ascension.

7. ## Re: Right ascension and declination - vs lat/long

Declination maps to Earth latitudes. If you are standing on the 30th parallel (30º North latitude) then a star located at Declination +30º will, at some point during the day, pass directly overhead through the zenith point.

RA is slightly more tricky. Earth longitude is based on 360º and the 0 point is an arbitrary position we assign. Since the earth is spinning, we can't really map longitude to sky positions. Instead the meridian lines in the sky are mapped to time offsets in hours, minutes, and seconds from a 0 point in space.

The zero point is chosen based on a fixed rule. It's the meridian where the sun will cross the equator at the moment of the spring (vernal) equinox. Since Earth has roughly a 23-1/2º tilt which is constant throughout the year ("axial precession" causes the Earth to wobble like a top except it takes about 26,000 years for one complete wobble On the scale of a human lifetime we can assume the Earth doesn't wobble at all.)

That means that this point in space (the RA 0 point) will be the same every year.

We can't use alt/az to describe the positions of stars because that's different for every observer at any different viewing location or on any other date or time. But the RA/Dec system is consistent... it always works. You can record these positions, hand them to someone else, who can then find that object on any day or time regardless of where they are on the planet.

8. ## The Following User Says Thank You to TCampbell For This Useful Post:

Lobster4221 (12-17-2012)

9. ## Re: Right ascension and declination - vs lat/long

The reason that RA increases to the east is so that, when you are watching the meridian over a period of time, the RA coordinates of objects that cross it increase as time passes.

Hour angle is used, but only for specific purposes. An object's hour angle is the length of time since the object last crossed the meridian. The hour angle of Polaris is very useful for setting up a polar scope to perform an accurate polar alignment.

The technical definition of an object's RA is the hour angle of the vernal equinox when the object crosses the meridian. This explains why the numbers increase to the east, and also explains why RA is measured in units of time, rather than in degrees.

10. ## The Following User Says Thank You to KathyNS For This Useful Post:

Lobster4221 (12-17-2012)

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