Total Solar Eclipse July 22, 2009


July 22, 2009 marks the longest eclipse of the 21st century.

Eclipse watchers know that weather frequently changes their plans at the last moment. All the detailed calculations for viewing the eclipse done months and years in advance are thrown into disarray.

circa is a calculator which computes the times the eclipse phases are visible at a given location. You can even use the iPhone 3G’s built in GPS to have these calculations done while you’re moving from one site to another!

Special eye protection is required to view the eclipse outside the time and path of totality. This software computes times for partial and total eclipse phases. If you are unsure when and where it is safe to view the eclipse, ask for help.

Eclipse Phases

Four phases describe a total eclipse: 1st through 4th contacts.

1st contact occurs when the moon begins to obscure the sun. This marks the start of the partial phase of the eclipse.

On the globe, the two furthest apart red lines show the northern and southern limits in which a partial eclipse is visible.

The three closely spaced red lines mark the path of totality. Only in this area can one experience a total solar eclipse, beginning with 2nd contact and ending with 3rd contact. During this time the moon completely obscures the sun, and the solar corona is visible. Finally, the end of the eclipse is known as 4th contact.

A little basic math allows one to compute the duration of totality (3rd-2nd contact) and the duration of the eclipse (4th-1st contact).

In some cases, when the sun would otherwise be below the horizon, the durations shown substitute the time of sunrise or sunset in place of the contact time. In these cases, the contact time is not shown. (The times for sunrise and sunset for the chosen location are always shown on the reverse side.)

Tap the displayed times to show the sun’s altitude at the time of each contact. Tap again to restore the time display.

The Globe

The light blue marker shows the chosen location.

Stop the globe spinning by tapping it. To start it again, tap twice quickly.

For a little fun, roll the world underneath your finger.