Date and Time

The positions of the Sun, Moon, planets, and stars change over the course of days, months, and years. Our units of time are intimately tied to astronomy.

Calendar History

The Egyptian calendar had exactly 365 days, but a year is actually 365.24219 days. Julius Caesar introduced leap years (every 4th year) in 44 BC. The Gregorian calendar (every 4th year, but not every 100th, except every 400th) replaced it in 1582.

Julian Date

The Julian Date (JD) is the total number of days elapsed since 12 noon at Greenwich on 1 January 4713 B.C. Julian dates start at noon Greenwich time and don't observe time zones. This uniform time scale simplifies astronomical calculations.

Universal Time and Time Zones

In 1884, the Earth was divided into 24 time zones. Universal Coordinated Time (UTC), also called Greenwich Mean Time (GMT), is the local time in Greenwich, England.

Solar vs. Sidereal Time

• Solar Day - 24 hours, based on the Sun returning to the same position.
• Sidereal Day - About 23 hours 56 minutes, based on the Earth's rotation relative to the stars.

The 4-minute difference occurs because Earth moves about 1° in its orbit each day.

Dynamic Time

Terrestrial Dynamic Time (TDT) is based on atomic clocks and is the standard for precise astronomical calculations, free from the irregularities of Earth's rotation.