gnomon —the upright part of a sundial that casts the shadow on the face of the dial.

Gregorian calendar —a revised calendar introduced by Pope Gregory in 1582 to correct inaccuracies that had crept in since Julius Caesar had introduced his calendar in 45 BCE . The Gregorian calendar is still used today. leap year —a year in which an extra day is added to the month of February, making it 29 days long rather than 28. This is done because a year cannot be divided into an equal number of days, as it is roughly 365¼ long. All the quarter days are added every fourth year at the end of February.

lunar month —the time between one new moon and the next. It is equal to 29 days, 12 hours, and 44 minutes.

sidereal day —the time it takes for the Earth to rotate once on its axis, measured by observing the positions of the stars. It is 4.09 minutes shorter than a solar day. Sidereal comes from a Latin word meaning star. solar day —the time it takes for the Earth to rotate once on its axis. It is measured as the time between two successive returns of the Sun to its highest point in the sky each day. solar year —throughout the year the Sun rises in slightly different places each day. A solar year is the time it takes for the Sun to return again to rise in the same place. It is the time it takes for the Earth to go around the Sun once and is equal to 365,242 solar days.


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