Following upon my presentation of a few historical resource, today I will note a few in the realm of Astronomy. Certainly a knowledge of those celestial objects in the heavens above is a worthy thing!
Image of the Mural Quadrant of Tycho Brahe (1601AD).
For interesting articles, a rather useful "This Week's Sky at a Glace" section, and reliably high quality charts and information, Sky and Telescope Magazine is worth being referenced, even if you don't subscribe to the magazine. You can find the site here:
Sky and Telescope
For information on passing satellites, comets, asteroids, and even details on the current location of the planets, Heavens Above is excellent. The site is not flashy, but certainly packed with information, and all can be tailored to the location of the reader (rather important, actually, if you want specific times and details). The site also allows you to create a map of the sky for any location at any time. Take note:
Those interested in space missions, and who appreciate magnificent photographs, should certainly take note of NASA's webpage!
For sunrise, sunset, and other information related to the length of days and sun data, the Gaisma website is superb. It is filled with rather useful charts for any city in its thorough database. Even if it is not your favorite subject, there is something nifty about a chart showing the precise path of the sun across your sky, and this compared to its path in mid-Summer or mid-Winter. It is found here:
Astronomy being so closely aligned historically and intellectually with Geometry, I close with a link to an excellent summary of the first four books of Euclid's Elements at Furman University's website. For those reading through the elements, this pictorally presents the text, line-by-line. Enjoy!
Euclid's Elements Books I-IV