PBS
Pbs Learning Media: Tidal Curiosities
This illustrated essay from the NOVA Web site answers questions about irregularities in the tides.
Exploratorium
Exploratorium: Total Solar Eclipse
This site provides detailed information about solar eclipses, including what to watch for and how to view an eclipse, why an eclipse occurs, and various feature stories.
NASA
Nasa: Space Place: What Causes the Seasons?
A fun resource that teaches students the reasons why Earth's four seasons happen they way they do.
CK-12 Foundation
Ck 12: Earth Science: Seasons
[Free Registration/Login may be required to access all resource tools.] Learn how and why seasons occur.
CK-12 Foundation
Ck 12: Earth Science: Solar Eclipses
[Free Registration/Login may be required to access all resource tools.] Find out how solar eclipses occur.
Other
On Hiatus: The Moon
This commercial site [without any advertising] provides a diagram of the current illumination of the Moon, and the dates of the most recent and upcoming New, Quarter, and Full Moon phases. All dates and times are in universal Time, only.
CK-12 Foundation
Ck 12: Physical Science: Orbital Motion
[Free Registration/Login may be required to access all resource tools.] Orbital motion and why it occurs, and the orbits of the Earth around the sun and the moon around the Earth.
CK-12 Foundation
Ck 12: Earth Science: Lunar Eclipses
[Free Registration/Login may be required to access all resource tools.] Discover how lunar eclipses occur.