Astrometry and Celestial Mechanics

Astrometry is the branch of astronomy that deals with the measurement of positions and motions of celestial objects.

Telescopes are the primary instruments used for astrometry, as they allow astronomers to precisely measure the positions and motions of celestial objects.

Aberration is the apparent motion of stars caused by the Earth's revolution around the Sun.

Nutation is the annual change in the orientation of the Earth's axis of rotation.

Precession is the long-term change in the orientation of the Earth's axis of rotation.

The substellar point is the point on the Earth's surface directly below a celestial object.

The ecliptic is the apparent path of the Sun across the sky over the course of a year.

The celestial equator is the imaginary line that divides the celestial sphere into two equal hemispheres.

Mercury has the shortest orbital period around the Sun, taking approximately 88 Earth days to complete one orbit.

Perihelion is the point in a celestial object's orbit where it is closest to the Sun.

Aphelion is the point in a celestial object's orbit where it is farthest from the Sun.

Neptune has the longest orbital period around the Sun, taking approximately 165 Earth years to complete one orbit.

The celestial axis is the imaginary line that connects the celestial poles.