# Relationship between ecliptic and celestial equatorial coordinate

### Celestial Coordinate System

SOFA, the IAU's Standards of Fundamental Astronomy, an accessible and authoritative set of algorithms and procedures that. For example, the celestial coordinates of the star Betelgeuse in the The points where the ecliptic crosses the equatorial plane of the celestial sphere are called. But bodies within the Solar System change their celestial positions. The most The angle of tilt is +23°26', which is called the obliquity of the ecliptic (symbol ε). To convert between ecliptic and equatorial coordinates, use the spherical triangle KPX. Next section: The relation between ecliptic and equatorial coordinates.

Equatorial coordinates are thus generally specified with an appropriate epochto account for precession. Popular epochs include J This conveniently divides the sky into the upper hemisphere that you can see, and the lower hemisphere that you can't because the Earth is in the way.

The pole of the upper hemisphere is called the Zenith. The pole of the lower hemisphere is called the nadir. The angle of an object above or below the horizon is called the Altitude Alt for short.

The angle of an object around the horizon measured from the North point, toward the East is called the Azimuth. Therefore, the Altitude and Azimuth of an object changes with time, as the object appears to drift across the sky.

In addition, because the Horizontal system is defined by your local horizon, the same object viewed from different locations on Earth at the same time will have different values of Altitude and Azimuth. Horizontal coordinates are very useful for determining the Rise and Set times of an object in the sky.

The Ecliptic is the path that the Sun appears to follow across the sky over the course of a year. It is also the projection of the Earth's orbital plane onto the Celestial Sphere.

• A Quick Guide to the Celestial Sphere
• Equatorial Coordinate System

The latitudinal angle is called the Ecliptic Latitude, and the longitudinal angle is called the Ecliptic Longitude. What do you think such a coordinate system would be useful for? If you guessed charting solar system objects, you are right! Each of the planets except Pluto orbits the Sun in roughly the same plane, so they always appear to be somewhere near the Ecliptic i.

The latitudinal angle is called the Galactic Latitude, and the longitudinal angle is called the Galactic Longitude. This coordinate system is useful for studying the Galaxy itself.

For example, you might want to know how the density of stars changes as a function of Galactic Latitude, to how much the disk of the Milky Way is flattened. Also, if Hour Angle is used in place of Right Ascension, then the Equatorial system is fixed to the Earth, not to the stars.

## Ecliptic Plane

Due to the movement of Earth around the Earth—Moon center of massthe apparent path of the Sun wobbles slightly, with a period of about one month. Due to further perturbations by the other planets of the Solar Systemthe Earth—Moon barycenter wobbles slightly around a mean position in a complex fashion. The ecliptic is actually the apparent path of the Sun throughout the course of a year.

This small difference in the Sun's position against the stars causes any particular spot on Earth's surface to catch up with and stand directly north or south of the Sun about four minutes later each day than it would if Earth would not orbit; a day on Earth is therefore 24 hours long rather than the approximately hour minute sidereal day.

Again, this is a simplification, based on a hypothetical Earth that orbits at uniform speed around the Sun.

### Celestial coordinate system - Wikipedia

The actual speed with which Earth orbits the Sun varies slightly during the year, so the speed with which the Sun seems to move along the ecliptic also varies. For example, the Sun is north of the celestial equator for about days of each year, and south of it for about days.

Axial tilt The plane of Earth 's orbit projected in all directions forms the reference plane known as the ecliptic. Here, it is shown projected outward gray to the celestial spherealong with Earth's equator and polar axis green.

The plane of the ecliptic intersects the celestial sphere along a great circle blackthe same circle on which the Sun seems to move as Earth orbits it. The intersections of the ecliptic and the equator on the celestial sphere are the vernal and autumnal equinoxes redwhere the Sun seems to cross the celestial equator. Because Earth's rotational axis is not perpendicular to its orbital planeEarth's equatorial plane is not coplanar with the ecliptic plane, but is inclined to it by an angle of about The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at these points, one from south to north, the other from north to south.

## Celestial coordinate system

Axial precession astronomy The orientation of Earth's axis and equator are not fixed in space, but rotate about the poles of the ecliptic with a period of about 26, years, a process known as lunisolar precessionas it is due mostly to the gravitational effect of the Moon and Sun on Earth's equatorial bulge. Likewise, the ecliptic itself is not fixed.

The gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earth's orbit, and hence of the ecliptic, known as planetary precession. The combined action of these two motions is called general precessionand changes the position of the equinoxes by about 50 arc seconds about 0.

Astronomical nutation Once again, this is a simplification. Periodic motions of the Moon and apparent periodic motions of the Sun actually of Earth in its orbit cause short-term small-amplitude periodic oscillations of Earth's axis, and hence the celestial equator, known as nutation. It is about Astronomers produce new fundamental ephemerides as the accuracy of observation improves and as the understanding of the dynamics increases, and from these ephemerides various astronomical values, including the obliquity, are derived.

Obliquity of the ecliptic for 20, years, from Laskar The red point represents the year