Celestial coordinate system?

Question

ARE RIGHT ASCENSION AND ECLIPTIC LONGITUDE SAME THING? IF NOT WHAT ARE THE DIFFERENCES?

Answer 1

right ascension
n. (Abbr. RA)
The angular distance of a celestial body or point on the celestial sphere, measured eastward from the vernal equinox along the celestial equator to the hour circle of the body or point and expressed in degrees or hours.

right ascension, in astronomy, one of the coordinates in the equatorial coordinate system. The right ascension of a celestial body is the angular distance measured eastward from the vernal equinox along the celestial equator to its intersection with the body's hour circle.

Right ascension
(RA): Celestial coordinate which is equivalent to the longitude of an object on Earth. The starting point for longitude on Earth is at Greenwich England, whereas in the celestial coordinate system it is at a point in the sky called the vernal equinox. The two great circles defined by the celestial equator and the ecliptic, which is defined as the plane of the solar system, cross each other at 2 points: at the vernal equinox (first day of spring) and the autumnal equinox (first day of autumn). The vernal equinox is the starting point for the RA coordinate system.

The noun right ascension has 2 meanings:

Meaning #1: (astronomy) the angular distance eastward along the celestial equator from the vernal equinox to the intersection of the hour circle that passes through the body; expressed in hours and minutes and second; used with declination to specify positions on the celestial sphere
Synonyms: RA, celestial longitude


Meaning #2: an arc of the celestial equator eastward from the vernal equinox

right ascension
Right ascension (abbrev. RA; symbol α = Greek letter alfa) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.

Explanation
RA is the celestial equivalent of terrestrial longitude. Both RA and longitude measure an east-west angle along the equator; and both measure from an arbitrary zero point on the equator. For longitude, the zero point is the Prime Meridian; for RA, the zero point is known as the vernal equinox point, which is the place in the sky where the Sun crosses the celestial equator at the March equinox.

RA is measured in hours, minutes, and seconds, eastward from the vernal equinox (sometimes it is also given in degrees). Being closely tied with sidereal time, it is both a unit of time and of angle. An hour of right ascension is equal to 15 degrees of arc, a minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc. An alternative measure, used in navigation, is Sidereal Hour Angle. The main difference being that RA is measured Eastward, and SHA is measured Westward.

RA can be used to determine a star's location and to determine how long it will take for a star to reach a certain point in the sky. For example, if a star with RA = 01:30:00 is at your meridian, then a star with RA = 20:00:00 will be in the meridian 18.5 sidereal hours later.

History
The concept of right ascension has been known at least as far back as Hipparchos who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchos and his successors made their star catalogs in ecliptical coordinates, and the use of RA was limited to special cases.

With the invention of the telescope, it became possible for astronomers to observe celestial objects in greater detail, provided that the telescope could be kept pointed at the object for a period of time. The easiest way to do that is to use an equatorial mount for the telescope, which allows the telescope to rotate at the same rate as the earth. As the equatorial mount became widely adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. The first star catalog to use right ascension and declination was John Flamsteed's Historia Coelestis Britannica (1712, 1725).

Ecliptic longitude
Ecliptic longitude (celestial longitude) is one of the co-ordinates which can be used to define the location of an astronomical object on the celestial sphere in ecliptic coordinates. In this system, the celestial sphere is divided into two hemispheres by the plane of the ecliptic.

The ecliptic is the path followed by the sun across the celestial sphere during the year. It crosses the celestial equator twice during the year, once at the Autumnal equinox and once at the Vernal equinox. There is no obvious point along the ecliptic at which ecliptic longitude should be zero, and so zero is arbitrarily defined as the location of the sun at Vernal equinox. Ecliptic longitude is then the angular distance of the object eastwards along the ecliptic from this point.

Ecliptic co-ordinates are most useful for solar system objects.

Ecliptic longitude is also known as Solar longitude. In ecliptic coordinate system, it is an angle between Earth and Vernal equinox.

Answer 2

They are different. Right Ascension is the number of degrees along the celestial equator from the vernal equinox. Ecliptic Longitude (also called Celestial Longitude) is degrees along the ecliptic from the vernal equinox. Since the ecliptic is inclined 23 1/2 degrees to the equator, the two are not exactly the same except at the two places where the equator and ecliptic cross. That is at 0 and 180 degrees, which are the vernal and autumnal equinoxes. Oh, they would probably be the same at 90 and 270 degrees too. But in between there would be small but varying difference between them.

Answer 3

No. They are similar and related, but not identical.

The ecliptic longitude of a planet is the longitude of the planet with respect to the plane of the Earth's orbit about the sun. The Earth's equator is tilted at about 23.45 degrees to the plane of its orbit. The plane of the Earth's orbit is the zero-plane in the case of ecliptic coordinates.

The right ascension of a planet is the astronomical longitude of a planet with respect to the plane of the Earth's celestial equator and the Vernal Equinox, (the point where the sun crosses the equator at the moment spring begins in the northern hemisphere), which serves as the astronomical prime meridian point. The plane of the Earth's equator is the zero-plane in the case of equatorial coordinates.

Answer 4

Yes, they are the same, but right ascension is time dependent. If you are looking at an ephemeris, make sure that the time and date are correct.