What is Time: HJD

Heliocentric Julian Date (HJD) is the Julian Date corrected for the differences in the Earth’s position relative to the center of the Sun.


What is Time: JD

Julian Dates (JD) are a continuous count of days, and fractions of days, since noon Universal Time on January 1, 4713 BC (on the Julian calendar)


What is an Epoch

An epoch is a particular time period, point in history, or a moment defined by a particular event, and is a moment of time used as a reference point for a time-varying astronomical quantity.

Basic Physics

Kepler’s Laws

Kepler's laws of planetary motion are: the orbits of the planets are ellipses with the sun at one focus, a line from a planet to the sun sweeps over equal areas in equal intervals of time, and a planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed


Astrometry Overview

Astrometry is the area that deals with the positions and motions of celestial objects. It has two main scientific objectives to provide: A stellar reference frame to which the motions of celestial objects (Solar System and beyond) may be referred Basic...


Telescopes: An Overview

Telescopes gather light from objects, and there are two main types: refractor (uses a lens to bend light) and reflector (uses a mirror to reflect light). Aperture and field of view are two main parameters of telescopes.