Companion Website Introduction Destinations

Here are our favorites for this chapter. Go ahead and check them out, but be sure to return back to do more exploring when you're done!

For a good selection of general sites of broad interest and utility that are related to many chapters of the book, see our collection of General Web Destinations For All Chapters.

List of Constellation Facts and Figures

Includes the stars the constellations contain, and a sky map too. Also at this site you will find The Constellations and Their Stars web page, maintained by Chris Dolan at the University of Wisconson-Madison Department of Astronomy. Here you will be able to generate a star map online. Other resources are listed in our General Web Destinations For All Chapters.

Primer on Seasons

See why the modern explanation of the pattern of the seasons fits important observable facts, and is not "just a theory."

Coordinate Conversions

Convert between right ascension/declination, galactic, and ecliptic coordinates, or find the coordinates of a particular object by name. The "name resolver" returns the coordinates for the given source name, and the search is made around those coordinates within a specified search radius. The object name is sent to one of two selectable astronomical databases for resolution.

Astrophysical Coordinates

All astronomical positions are measured on the sky in spherical coordinates. That is, we use some form of latitude, longitude measure for location. Just as on the Earth you can use either the axis of rotation or the magnetic pole as your reference, on the sky there are a number of alternative reference systems. The purpose of this document is to provide a brief background on the definitions used in astronomy, and a starting point for those who are new to the subject.

Celestial Spheres

A celestial sphere shows directions in a coordinate system analogous to Earth's latitude and longitude projected up into the heavens

Stardial

Stardial is a stationary weather-proof electronic camera for recording images of the sky at night. Like a sundial, Stardial is a stationary device placed outside 24 hours a day. Both depend on and make evident the rotation of the Earth. A sundial operates with sunlight, while Stardial operates with starlight. There the similarity ends. Stardial's purpose is to record images of the night sky and provide them in real time to the world wide web. These images can be used for a variety of purposes, including visualization of geosynchronous satellites, monitoring the daily apparent motion of the stars, looking for dim or variable objects such as variable stars or comets, or just "looking up."

What Time Is It?

Until recently, when atomic clocks became available, time was reckoned by the Earth's motions: one rotation on its axis was a "day" and one revolution about the Sun was a "year," an hour was one twenty-fourth of a day, and so on. The position of the Sun in the sky was used to measure various intervals. Now there are many methods used to keep time, each having its own special use and advantage. Learn about the differences between local time, apparent time, mean time, civil time, universal time, international standard time, sidereal time, and ephemeris time from the discussion given at this site.

Solar System Live

Caution, slow link from non-European sites! An interactive orrery on the Web. You can view the entire Solar System or just the inner planets through the orbit of Mars, using either realistic or a choice of compressed scales. The controls allow you to set time and date, viewpoint, observing location, orbital elements to track an asteroid or comet, and a variety of other parameters. You can compose a request with custom settings and save the results in your browser's hotlist or bookmark table, allowing direct access with all the controls preset to your own preferences. To use this page, you need a graphical Web browser with forms support and the ability to display GIF images.

Deep Space Network

NASA's Deep Space Network is the largest and most sensitive scientific telecommunications system and the most precise radio navigation network in the Solar system. Its principal responsibilities are to support interplanetary spacecraft missions and radio and radar astronomy observations in the exploration of the solar system and the universe. Find out how this network is used for modern spacecraft navigation and distance measurement.

Kepler's Laws Java Orbit Simulators

Provided by NASA's Observatorium,the Orbit Simulator applets at this site illustrate the relationship between orbital period and altitude for a circular orbit around the earth. You can click and drag the satellite to any altitude and see how the orbital period changes in accordance with Kepler's Laws. For an introduction to some of the history behind the modern study of planetary orbits, see also the pages on Kepler, Galileo and Newton in the Astronomy Picture of the Day Archive.

The Moon Phase Page

Delivers an image of the current phase of the Moon as viewed from the central US (Mountain time). This page can also be used to view the Moon's phase for any date that you select. See also the similar and well-done Phase of the Moon page of Lunar Outreach Services.

Sundial Tutorial

The earliest known sundial, found in Egypt, dates from at least 3,500 years ago, although crude versions must have been in use for even longer. As the Earth rotates on its axis, the Sun appears to move uniformly across the sky. If a rod is placed parallel to the Earth's axis, its shadow will naturally move uniformly around itself. In other words, as the Sun appears to move, due to the Earth's rotation, through the sky at a rate of about 15 degrees per hour, so will the shadow move at the same rate. This is the principle on which most (but not all) sundials are based. Learn more, including the history of the variable length of "hours" in a day, via this site.

Eclipse Tutorial

An eclipse occurs when a body cuts off the light from a light source so that we can no longer see it shining. An eclipse can be due either to a dark body coming between us and a light emitter, so that we can no longer see the source, or it can be a body coming between a light source and the body that the light is illuminating, so that we no longer see the illuminated body. Learn more from this site, including the conditions for a solar or lunar eclipse.

Solar Eclipse Path Predictions

Includes ASCII text files for all central solar eclipses from now through the early portion of the next decade, and complete NASA eclipse bulletins available 18 to 24 months before each eclipse. The full bulletins contain detailed predictions, maps and meteorological data for future central solar eclipses, and the site includes charts and animations of the predicted eclipse paths. See also solar eclipse predictions available in a different form from Catania Astrophysical Observatory (Italy).

Primer on Archaeoastronomy

One of the fundamental objectives of archaeoastronomy is to find an alignment at a site. Archaeoastronomers look for pairs of stones or architectural features, located at some distance from each other. When they find a pair, the alignment between these features and celestial objects can be checked.

Astronomiae Historia / History of Astronomy

By far the most comprehensive and academically rich source of information on topics related to the history of astronomy on the Web, this site is a tour de force. Included are index entries, for example, on the topics of Archaeoastronomy and Ethnoastronomy, Astrology, Mythology, Religion, Astrophysics, Calendars, Time and Chronology, Celestial Mechanics, Constellations, Maps and Atlases, Cosmology, Extragalactic Astronomy, Instruments, Nebulae and Star Clusters, Space Exploration, Stellar Astronomy, World Systems, and many other topics. Recommended.

The Metonic Cycle

The Greek astronomer Metos, in the fifth century BC, discovered that the dates of the phases of the Moon repeated exactly (well, very close to exactly after a period of 19 years. To be precise, close inspection shows that 19 tropical years contain 6939.60 days, while 235 synodic months contain 6939.69 days. It is therefore possible for a series of four or five eclipses to occur on the same dates 19 years apart. The "metonic cycle" was used to determine how intermediate months could be inserted into a lunar calendar so that the calendar year and the tropical (seasonal) year were kept in step. This cycle was almost certainly known to the ancient Babylonians and was possibly used by Thales around 585 BC according to this page, which you can explore for more information on this topic.

Celestial Instruments

Browse through some of the historical instruments on display at the University of Bologna. The instruments used by the Bolognese astronomers from the early 18th century to mid-19th century have been brought back to their original rooms and photographed in place as they would originally have been used.

Scientific Method FAQ

Frequently Asked Questions on the scientific method. This useful primer covers the difference between a fact, a theory, and a hypothesis and explains the basic ground rules for scientific investigation.

Galileo Galilei

Galilei was born in Pisa in 1564, the son of Vincenzo Galilei, well known for his studies of music, and Giulia Ammannati. He studied at Pisa, where he later held the chair in mathematics from 1589 - 1592. He was then appointed to the chair of mathematics at the University of Padua, where he remained until 1610. It was at Padua that he made his most famous dicoveries. Read and see more in this excellent biography from a site based in Florence, Italy.

Making A Model Of The Solar System

This page provides many links to sites that will help you build and better understand the scale of our solar system.  If you decide to build your own scale model you will find that you may need more than a pencil and tape measure. You may also need to use other instruments, like the odometer in your car!

Laws of Science Abused by Science Fiction

It is possible to learn a lot about the laws of science by attempting an informal feasibility study on, for example, a spacecraft from a science fiction comic book, a sci-fi magazine cover, or a STAR WARS or STAR TREK video. This site gives some of the laws of science that are frequently violated by artists and authors. Descriptive examples of such errors are given and studied. This page is part of the NASA Multimedia Space Educators' Handbook.

Keplerian Elements Tutorial

Do you want to go beyond the absolute basics and understand in a bit more detail how we keep track of the exact locations of the kazillions of satellites and other objects in orbit around the Earth (and elsewhere)? Then this tutorial is highly recommended. It gives a straightforward introduction to the vocabulary and concepts, and even manages to make thinking about such things in three dimensions seem somewhat easy.

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