An astronomical constant is a physical constant used in astronomy. The astronomical constants include units of astronomical distance (the astronomical unit (A.U.), the light year (ly), and the parsec (pc)), units of time (the sidereal year, the tropical year, and the Gregorian year), and various values for distances, masses, etc. (e.g., the mass and the mean radius of the Earth, and the mass, radius, and luminosity of the Sun). The astronomical constants may also be taken to include such physical constants as the speed of light (c), the gravitational constant (G), and the Planck constant (h). In science, a physical constant is a physical quantity whose numerical value does not change. ... Astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and monitoring of transient phenomena. ... The astronomical unit (AU or au or a. ... A light year, abbreviated ly, is the distance light travels in one year: roughly 9. ... The parsec (symbol pc) is a unit of length used in astronomy. ... The sidereal year is the time for the Sun to return to the same position in respect to the stars of the celestial sphere. ... A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... Cherenkov effect in a swimming pool nuclear reactor. ... According to the law of universal gravitation, the attractive force between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. ... Plancks constant, denoted h, is a physical constant that is used to describe the sizes of quanta. ...

The values of the astronomical constants have been recalculated at various times, perhaps most notably by the Canadian-born astronomer Simon Newcomb (1835–1909). Main articles: History of Canada, Timeline of Canadian history Parts of Canada have been inhabited by aboriginal peoples (known as First Nations) for at least 40,000 years. ... An astronomer or astrophysicist is a scientist whose area of research is astronomy or astrophysics. ... Simon Newcomb. ...

External links

• Tables of constants (http://www.astronomynotes.com/tables/tablesa.htm) — from Nick Strobel's Astronomy Notes (http://www.astronomynotes.com/)
• Astronomical constants index (http://www.jqjacobs.net/astro/astro.html) — James Q. Jacobs