WWW Starting Points

The Astronomy Cafe: Maintained by a working astrophysicist. Includes the Ask an Astronomer section, with an archive of hundreds of previously asked and answered questions.
Definitions of Astronomical Terms: Brief and to the point. See also the Space Flight Glossary and John Wallin's Astro 106 Glossary.
Mount Wilson Online Postscript Star Map Service: This service allows you to create a high-resolution Postscript file that contains a customized star map for the location, time, and conditions you specify. The resulting file is returned to you via the Web. This file contains printer control commands formatted for a Postscript printer, so you must have appropriate software on your computer to view it, or send it to a Postscript printer to create a hard copy. Links are provided to help you obtain appropriate software if you need it.
Make a Planisphere!: This planisphere service allows you to get a chart of visible stars and planets from any point of view on Earth in either Compuserve GIF or Postscript format.
What's Happening in the Heavens: The Old Farmer's Almanac offers the dates and locations of solar and lunar eclipses for the year, as well as a user-selectable monthly listing of upcoming celestial events, the days of the full Moon for the next five years, and a complete set of Moon phase predictions (useful for determining dates near a new Moon that will therefore be best for deep-sky observing). Check it out here, then go outside and look up!
The Skywatcher's Page: A collection of resources and links for people who watch and gaze at the night sky, from Abrams Planetarium. Of special interest is their Skywatcher's Diary, which contains a list of celestial events and observing targets of interest for each evening (or morning) in the current month. Those in the UK may wish also to look over the Forthcoming Astronomical Events page provided by the Norwich Astronomical Society.
Binocular Observing: Gives a list of suggested objects to observe, a short introductory lecture, and a list of sites to visit to learn more about the selection and use of binoculars for astronomical observing. See also the related Observing Guide. For observing with even less equipment, see for example Beginners on this page.
Woodlands Sky Almanac: Caution, slow link from the Americas and Europe! Provides online star charts for all regions of the globe in high quality graphics. If you're interested in learning constellations, or viewing the night sky, this is a great place to start. See also their SkyAlmanac Calendar of celestial events.
USA and World Weather: This commercially run but freely accessible site has 4-day forecasts, satellite images, and city-specific weather information for a large number of locations throughout the globe, and even an almanac of typical weather that can be expected for different months of the year. Check the weather before you go out to observe!
Date and Time Gateway: Figure out the date and time in any of the defined time zones of the world, and check the time and time zone for a large number of major cities throughout the globe. (Scroll down to the bottom of this page when you get there to find regional time zones for the U.S.) For sunrise and sunset computations, as well as other useful information such as the times of Moon phases during the year, see also the USNO Sunrise/Sunset/Twilight and Moonrise/Moonset/Phase Computation Page from the U.S. Naval Observatory.
Solar System Physical Calculator: This Javascript calculator has several physical constants as well as the masses, radii, and other dimensional features of the major objects in the solar system built-in! It works best on Unix computers equipped with advanced Web browsers, but should work acceptably on Mac and Windows-compatible machines as well if you use a Java-capable browser. Quite useful. To understand the numerical constants and units programmed into this page, see the accompanying list of Physical Constants and Astronomical Data.
The Science Reference Shelf: A listing of sources for fundamental physical constants, conversion factors, common units of measure, chemical safety, and a variety of useful categories. See also the 1986 CODATA Recommended Values of the Fundamental Physical Constants at the National Institute of Standards and Technology.
NASA Acronyms: Having trouble figuring out what in the world a WFPC might be? Can't figure out the difference between XRS, XPS, XSAT, XSS, XSW, and the XTE? Here's searchable help from the folks at GSFC (the Goddard Space Flight Center). Requires a forms-capable browser.
Science Question of the Week: Check here for a new science question-and-answer every week.
Ask an Astronaut: Explore this site for text and multimedia files featuring the astronaut of this month, and then send in your questions. The answers to selected questions are posted and can be reviewed online.
Space Mathematics: Several years ago NASA created curriculum supplements for several high school courses, including a supplementary publication titled "Space Mathematics, a Resource for Teachers" published in 1972 for teachers of mathematics. This site contains an online version of the recently updated supplement that should be of general use, covering mathematical aspects of some space missions, Algebra, Geometry, Trigonometry, Conic Sections, and applications from other related subjects.

Web Sites for Print Media

Mercury Magazine: Mercury is the bimonthly publication of the Astronomical Society of the Pacific. Written for the motivated nonspecialist, this magazine features nontechnical articles on astronomy research, education, history, and public policy. The Web site includes a searchable index of past issues and links to online material.
Astronomy Magazine: A popular magazine with much explanatory and newsworthy material aimed at the beginning to intermediate astronomer. The Web site includes pages on sky events, clubs, equipment, books, software, news, and other related links.
SKY Online: Provided by the publishers of Sky and Telescope Magazine and CCD Astronomy. This magazine is aimed at a slightly more experienced level than other beginning astronomy magazines, and is read by many professional instructors and astronomers. The Web site includes a broad range of online resource material from calendar information and news to tips for choosing a telescope and accessories. Make a bookmark in your hotlist for their Weekly News Bulletin.
Astronomy Now: Britain's leading astronomical magazine. The Web site includes news updates, astronomical events, a picture gallery and a list of upcoming events for astronomy clubs in Europe and the UK.

Picture Source Sites and Updates

Astronomy Picture of the Day: If you have a good network connection, this is an interesting place to check in regularly. The image of the day is posted along with a nice explanation (sometimes a bit on the short side). Links to other resources are frequently given. The site tries to showcase the work of leading-edge scientists, and consequently the significance of these images is usually pretty high.
Latest HST Pictures: The latest, most up-to-date pictures from the Hubble Space Telescope at the Space Telescope Science Institute (STScI), with explanations. The place to go for the latest Hubble images. Also see their index of images organized by subject.
The STScI Digitized Sky Survey: Produced by the Space Telescope Science Institute using images from several sky surveys, this extensive archive can return images corresponding to nearly any user-selectable location in the sky in either FITS or GIF format. You can also use it to obtain celestial coordinates for a particular object based on the object's name or designation. Caution, many of the images contained in this survey are copyrighted.
Astronomical Images from the Anglo-Australian Observatory: Caution, slow link from the Americas and Europe! The primary online repository for images taken by the renowned deep-sky photographer David Malin. Despite its title, the site contains pictures taken using several different telescopes at the AAO and elsewhere. Includes many beautiful color pictures. (Note also the copyright.)
The Hottest Messier Stuff Available: A really well-organized and usable page of links and images at the Web site of Students for the Exploration and Development of Space. Definitely worth a look! See also their Space Images Archive, which has a graphics-heavy initial page but is searchable.
ASP List of NGC Images: A very long list of New General Catalog images assembled by the Astronomical Society of the Pacific. Some images are copyrighted, so check before using them.
National Geographic-Palomar Observatory Sky Survey: This site, not for the timid, provides searchable access to an on-line astronomical catalog of images of millions of stars and galaxies obtained from the Automated Plate Scanner (APS) digitized National Geographic-Palomar Observatory Sky Survey (POSS I) plates. The catalog entries include each object's classification, magnitude, position, color, and many other image parameters.
Cookbook Camera Image Archive: A pretty amazing set of images, all taken by amateurs using a variety of equipment and home-built CCD kit cameras. Although this is a commercial site (and for fairness we should mention one provided by a leading alternative vendor), the pictures here are very much worth the trip.

Planetarium Links

The Planetarian Journal: The Planetarian is published quarterly by the International Planetarium Society. The journal's Web site has information and an online repository containing a sample of articles that have appeared in past issues.
ASP List of Planetariums: Another useful list from the Astronomical Society of the Pacific.
Cosmic Mystery Tour: Sort of a "planetarium show on the Web," this site gives an introduction to Big Bang cosmology and present ideas regarding the origins of the universe. Part of the related Cosmos in a Computer portion of the online "Science for the Millennium" Expo at NCSA.