astronomical information center

Glossary of Terms

• Astronomical Almanac Online Glossary

Phenomena of the Sun and Moon

• Rise, Set, and Twilight Definitions
• Lengths of Longest and Shortest Days
• Length of Day and Night at the Equinoxes
• Sunrise and Sunset Times Near the Solstices
• Visibility of the Crescent Moon
• References on Eclipses
• Phases of the Moon and Percent of the Moon Illuminated

Time

• Universal Time and Greenwich Mean Time
• What is Terrestrial Time?
• Computing Greenwich Apparent Sidereal Time
• What are the U.S. Time Zones?
• World Time Zone Map
• When Does Daylight Time Begin and End?
• The Equation of Time

Calendars and Historical Events

• Introduction to Calendars
• Leap Years
• U.S. Holidays
• The International Date Line
• The 21st Century and the 3rd Millennium
• The First Sunrise of the New Year
• Spring Phenomena, 25 BCE to 38 CE

Computing Astronomical Data and Phenomena

• Converting Between Julian Dates and Gregorian Calendar Dates
• Computing Times of Rise and Set
• Approximate Solar Coordinates
• Computing Greenwich Apparent Sidereal Time
• Computing Altitude and Azimuth

Celestial Navigation

• Celestial Navigation Algorithms
• Celestial Navigation Resources

Other Topics

• Astronomical Data Used for Litigation
• The Seasons and the Earth's Orbit
• Variation in Times of Perihelion and Aphelion
• International Celestial Reference System (ICRS)
• When Did the Asteroids Become Minor Planets?

Links to Information at Other Sites

• "Explaining The Moon Illusion" - why the Moon looks larger close to the horizon   (by Lloyd Kaufman and James Kaufman for the Proceedings of the National Academy of Sciences)
• What is a Blue Moon?   (by David Harper)
• An Introduction to Green Flashes   (by Andrew T. Young, San Diego State University)
• Photographs of an Inferior-Mirage Sunset - strange Sun shapes near the horizon and related phenomena   (by Andrew T. Young, San Diego State University)
• Sunspots and Solar Activity
• Weather and Climate
• Sky conditions for astronomical observing
• Tides
• Maps