Home > First Point of Aries

In the equatorial coordinate system the First Point of Aries is defined to have a right ascension of zero. The declination is also zero due to the position on the celestial equator.

It is important to keep in mind the geocentric nature of this definition, and how things can seem backwards when thinking solar-centrically, as when trying to use orbital elementsThe elements of an orbit are the parameters needed to specify that orbit uniquely, given a model of two ideal masses obeying the Newtonian laws of motion and the inverse-square law of gravitational attraction. Because there are multiple ways of defining a. For instance, Earth reaches perihelion shortly after the hibernal solsticeSolstice is an astronomical term regarding the position of the Sun in relation to the celestial equator. It is related to the axial tilt of the planet, and not the Solar apsides ( aphelion and perihelion) of the planet, as many people commonly believe., three-fourths of a year after vernal equinox. With all the vernal-equinox talk, it's easy to think Earth's longitude of perihelion should be about 270°. But longitude 0 is the far side of the Sun at vernal equinox -- i.e. Earth won't reach longitude 0 until the autumnal equinox, and so the longitude of perihelion is more like 90° (about 103° in fact.) Sidereal clocksSidereal time is time reckoned by sidereal days and is the time elapsed since a transit of the vernal equinox; this is, strictly, apparent sidereal time''. Mean sidereal time is reckoned not from the actual transit of the vernal equinox, but rather from t agree with conventional clocks at the autumnal equinox intead of the vernal for similar reasons.