CYCLE	DEFINITION	APPROXIMATE	EXACT	MULTIPLE
tropical year	period from equinox to equinox	365.25 days	365.2422 days
synodic month	period from new moon to new moon	29.5 days	29.530589 days
sidereal month	period for moon to return to the same position on the celestial sphere	27.3 days	27.32166 days
draconic month	period for moon to return to the same node	27.2 days	27.21222 days
3 sidereal months	period for moon to return to same position on the celestial sphere at the same time of night	82 days	81.96498 days	3 x 27.32166
lunar standstill	period from full moon to full moon at maximum angular distance from the sun	18.6 years	18.61 years
Metonic cycle	number of lunar months which equal an integral number of solar years	235 months	6939.688415 days	12 x 12 mo. + 7 x 13 mo.
saros cycle	period between eclipses at same node	18 years	6,585.32 days	223 synodic + 242 draconic
Venus year	period spanning two appearances and two disappearances of Venus	584 days	583.9 days	263+50 +263+8
solar cycle	Number of years for calendar to repeat same month and day sequence		28 years
Julian cycle	Used to fix the beginning of the Julian Day		7,980 years	19 (Metonic cycle) x 28 (solar cycle) x 15(indiction cycle)
Julian Day	Number of days since January 1, 4713 B.C.	2,451,927	2,451,927.58333 (7:00 pm PDT)