The celestial sphere is the imaginary globe that surrounds us. The ancients believed that the stars were fixed. Credit: Holley Y. Bakich
Today I want to talk about the equatorial coordinate system. Start with this: what happens if we could project the coordinates of the land of longitude and latitude in the sky? Latitude would not be a problem, but longitude would change continuously because of the rotation of the earth. Somehow, we must attack the longitude to heaven.
If we project the equator and the poles of the earth on the celestial sphere, we get the celestial equator and the North and the South Celestial Poles. The circles through both poles are perpendicular to the celestial equator. To find the position of an object in the sky, imagine a circle that passes through the poles and the object. This is the circle of the hours of the star and corresponds to a longitude meridian on earth.
Right ascension and declination
The first coordinated in the equatorial system is the declination, the corner between the object and the celestial equator (measured along the time circle of the object). It varies from 0 ° for an object on the celestial equator to 90 ° North or South. We measure it in degrees, minutes (1/60 of degree) and seconds (1/60 minutes). So, the declination is easy. The other coordinated, the right ascension, is not.
To set the zero point of the right ascension, astronomers use the intersection of the Earth’s equator and its orbital plan, the ecliptic. This is called March (or Vernale), but you could see it called the first point of Aries. While the earth orbit the sun in orbit, the sun seems to move every year around March 21, crossing the celestial equator that moves from south to north.
So when we measure the corner between the spring equinox and the time circle of the object, this is its right ascension. It is measured in hours, minutes and seconds. Yes, hours, not like the declination. Sometimes, astronomers do things just to be different. Since one day is 24 hours, even the rim of right ascension is 24 hours, every hour corresponding to 15 °. The right ascension is measured from the west to the east starting from the spring equinox, which becomes the starting point, 0 hours.
Things are moving
As for the right ascences and the variations of celestial objects for the purposes of amateur astronomy, some coordinates change and others do not (except for very long periods of time). The objects whose coordinates change include the sun, the moon, the planets, the asteroids and the comets. The objects whose coordinates can be said in order not to change include stars, nebulae, clusters and galaxies.
Due to small changes in the rotation axis of the earth, mainly caused by the gravitational shots of the sun and the moon, the spring equinox slowly changes its position, therefore also the system of equatorial coordinate is slowly changing over time. To take this into account, astronomers specify an era (a time of time) to which the coordinate system is referenced. The current sources use Epoch 2000.0, the beginning of the year 2000 DC
Finally, here is something that connects latitude and the declination of a star. In the northern hemisphere, if the declination of a star is greater than 90 ° degrees less your latitude, that star will never be fixed. We call it a circuit star. In the southern hemisphere, if the declination of a star is less than the less 90 ° latitude, the star will be circumpool.
