From Citizendium, the Citizens' Compendium
- Astronomy : The study of objects and processes in the observable universe, e.g. stars, planets, comets or asteroids.
- Planet : A cosmic body orbiting a star.
- Earth : The third planet from the Sun in our solar system; the only place in the universe known by humanity to harbor life.
- Calendar : A method used for keeping time on a scale of years.
- Month : Unit of time originally corresponding approximately to one cycle of the moon's phases, or about 30 days or 4 weeks.
- Week : Time unit equal to a number of days, now usually seven calendar days, previously weeks of between 4 and 20 days have been used historically in various places.
- Day : Defined as 86,400 seconds, using the SI definition of second.
- Equinox : Add brief definition or description
- Aphelion : Add brief definition or description
- Perihelion : Add brief definition or description
- Leap year : Add brief definition or description
- Light day : Distance that light travels in a vacuum in one day; 1 light day = 25,902,068,371,200 m = 2.5902067 * 1013m.
- Light hour : Distance that the light travels in vacuum in one hour, 1.0792528 * 1012m
- Light minute : Distance that light travels in vacuum in one minute; 17,987,547,480 m = 1.7987547 * 1010m.
- Light second : Distance that light travels in vacuum in one second; 2.99792458 * 108m.
- Light week : Distance that light travels in a vacuum in one week; 1 light week = 1.81314469 * 1011km.
- Light year : Distance that light travels in vacuum in one year; 9,460,730,472,580.800 km = 9.4607304 * 1012 km.
- Mathematics : The study of quantities, structures, their relations, and changes thereof.
- Perennial plant : A plant that lives for more than two years.
- Sundial : An instrument for measuring time, using the position of the sun in the sky.