Celestial Sphere
Celestial Equator - The celestial equators is the projection of the Earth's equator onto the sky. The declination coordinate is an angle measured with respect to the celestial equator.
Celestial Sphere - How do astronomers keep track of the sky when the earth is constantly moving? They use star charts that are maps of different regions on the celestial sphere.
Celestial Sphere - It is useful when discussing objects in the sky to imagine them to be attached to a sphere surrounding the earth. This fictitious construction is called the celestial sphere.
Celestial Sphere Java Applet - What you see in the sky depends on your location on the Earth. Use this site to create view of the sky for your location.
Precession of the Earth's Rotation Axis - The Earth's rotation axis is not fixed in space. Like a rotating toy top, the direction of the rotation axis executes a slow precession with a period of 26,000 years.
Equinoxes and Solstices - The Royal Observatory of Greenwich provides this explanation of the equinoxes and solstices.
Seasons - There is a popular misconception that the seasons on the Earth are caused by varying distances of the Earth from the Sun on its elliptical orbit. This is not correct. One way to see that this reasoning may be in error is to note that the seasons are out of phase in the Northern and Southern hemispheres: when it is Summer in the North it is Winter in the South.
What causes seasons? - The seasons have nothing to do with how far the Earth is from the Sun. If this were the case, it would be hotter in the northern hemisphere during January as opposed to July. Instead, the seasons are caused by the Earth being tilted on its axis by 23.5 degrees.
Precession- Due to the interaction between the Earth and the Moon, the rotation axis of the Earth is not stationary. The interaction arises because the Moon is not in the equatorial plane of the Earth; there is a misalignment between the spin axis of the Earth and the orbital axis of the Moon.
The Precession of the Earth's Axis- What does this motion tell us about the Earth's motion in space? If you ever had a spinning top, you know that its axis tends to stay lined up in the same direction--usually, vertically, though in space any direction qualifies.
Powers of 10- View the Milky Way at 10 million light years from the Earth. Then move through space towards the Earth in successive orders of magnitude until you reach a tall oak tree just outside the buildings of the National High Magnetic Field Laboratory in Tallahassee, Florida.
Coordinate Systems
Altitude and Azimuth - Two coordinates specifications suffice to locate an object visible in the night sky: its altitude above the horizon and its azimuth (location on the circle of the horizon).
Angular Measurement - Angular measurement is the process of determining the angle between two points, within the context of a particular coordinate system.
Astronomical Coordinate Systems - Find out what Coordinate Systems are used in astronomy. Horizontal, Equatorial, Ecliptic, and Galactic coordinates are discussed.
Astronomical Coordinate Systems - Let the Students for the Exploration and Development of Space (SEDS) walk you through the different kinds of coordinate systems used in astronomy.
Celestial Coordinate System - This site provides graphics that will help you understand the right ascension and declination celestial coordinate system.
Latitude and Longitude - Any location on Earth is described by two numbers--its latitude and its longitude. If a pilot or a ship's captain wants to specify position on a map, these are the coordinates they would use.
Right Ascension and Declination - Right Ascension (abbreviated R.A.) and Declination (abbreviated Dec) are a system of coordinates used by astronomers to keep track of where stars and galaxies are in the sky. They are similar to the system of "longitude" and "latitude" used on the Earth.
Sky Coordinates - This web page provides a good description of the different coordinate systems used in astronomy.
Sun or Moon Altitude/Azimuth Table - This page provides a way for you to obtain a table of the altitude and azimuth of the Sun or Moon during a specific day, at a time interval that you specify. Simply specify the object, date, tabular interval, and place below and click on the "Compute Table" button. The altitude and azimuth values are tabulated as a function of the standard time of the place requested (daylight time is not used) on a 24-hour clock. This form is provided by the USNO.
Time and the Sky Calendar
Calendars - There are two basic sources for calendars presently in use: the monthly motion of the Moon (Lunar calendars) and the yearly motion of the Sun (Solar Calendars). Read about the different calendars: Roman, Julian, Gregorian, etc.
Daylight Saving Time - The origin, history, and practice of Daylight Saving Time from Benjamin Franklin to the present.
How Time Works - Howstuffworks provides information about Time's Origins, Clocks, Time Zones, Daylight-Saving Time, the Calendar, B.C. and A.D.
Leap Year - Find out how leap years came about, and how a certain year is determined to be a leap year. This site was provided by the Royal Greenwich Observatory.
Names of the Months - This site provides information about the origin of the names of the Months.
North American Time Zones and GMT - This site gives the time difference between GMT and local time for both standard time and summertime (Daylight Savings Time), so you can convert to local time from GMT.
The Month - What is the connection between the moon and the month?
The Official U.S. Time - The Time and Frequency Division, part of NIST's Physics Laboratory, maintains the standard for frequency and time interval for the United States, provides official time to the United States.
Time Service Department - The Official Source of Time for the Department of Defense (DoD) and the Global Positioning System (GPS), and a Standard of Time for the United States
Time Zones - This site provides background information and explanations for Time Zones.
Timekeeping - Historically, the regular motion of objects in the sky served as the basis for timekeeping. Read about sidereal days and solar days as well as time zones and universal time.
What is Universal Time? - The answer to this question is provided by the US Naval Observatory Astronomical Applications Department.
Constellations
Constellation Index - At this site you can look at images of constellations, find out how constellations got their names, and find out how to read a star charts.
Constellation Information - View the constellation patterns and learn a few of the names of the bright stars in each constellation using this web site.
Constellation List - This site contains diagrams of the 88 constellations allow with some history behind their names.
Constellations - This site provided by the Students for the Exploration and Development of Space (SEDS) contains links, history, and star list for the 88 constellations.
Constellations - Historically, constellations were groupings of stars that were thought to outline the shape of something, usually with mythological significance. There are 88 recognized constellations, with their names tracing as far back as Mesopotamia, 5000 years ago.
Zodiac Constellations - Each year the Sun moves eastward in a complete circle around the sky. The path followed by the Sun is called the ecliptic, and any constellation containing the ecliptic is called a zodiac constellation.
Powers of 10- View the Milky Way at 10 million light years from the Earth. Then move through space towards the Earth in successive orders of magnitude until you reach a tall oak tree just outside the buildings of the National High Magnetic Field Laboratory in Tallahassee, Florida.
Star Charts
Free SFA Star Charts - This site contain free PDF star charts that users can print using their home computers. The four main star chart cover the entire celestial sphere.
SkyMaps - Free Monthly Sky Maps. The Evening Sky Map is suitable for all stargazers including newcomers to astronomy.
Star Charts from the AAVSO - Charts for professional and amateur variable star observations provided by the American Association of Variable Star Observers.
Star Maps - Locating stars and constellations on the celestial sphere is facilitated by a star map.
Your Sky - This is an the interactive planetarium of the Web. You can produce maps in the forms described below for any time and date, viewpoint, and observing location.
Star Names
A Skywatcher's Pronunciation Guide - We've often heard people ask, "How do you pronounce that?" This guide provides phonetic pronunciations for hundreds of stars and other astronomical objects.
Can I buy a star? - There is no place where you can purchase a star. There are a few businesses which claim to sell or name stars, but the names they give are not recognized by anyone in the scientific community.
Common Star Names - This site provides a list of common star names along with some history behind the names of stars.
Naming Stars - Get answers to you questions about how stars are named from the International Astronomical Union (IAU).
Naming the Stars - The stars on the celestial sphere are named in several different ways. As a result, the brighter stars may have more than one name. The web site gives a brief overview of naming stars.
Star Pronunciation Guide - You can also listen to Deborah Byrd pronounce the words and tell you a little bit about the objects they refer to (via Real Audio).
Software Directories
Astronomy Software Collection - This web site contains over a hundred links to astronomy software for Windows, DOS, Macintosh, and Unix operating systems. Links to freeware, shareware, and public domain software are included. The list includes links to planetarium programs, image processing, planet rendering, and much more.
Contemporary Laboratory Experiences in Astronomy - Contemporary Laboratory Experiences in Astronomy (CLEA) has developed laboratory exercises that illustrate modern astronomical techniques using digital data and color images. They are suitable for high- school and college classes at all levels, but come with defaults set for use in introductory astronomy classes for non-science majors. Each CLEA laboratory exercise includes a dedicated computer program, a student manual, and a technical guide for the instructor. The technical guides describe file formats, user-settable options, and algorithms used in the programs. The most advanced CLEA labs run under Windows on PC's, or on color-capable Macintosh computers.
Planetarium Software - Here is some information about various planetarium and sky simulation programs.