Beginner's Corner

Solar Eclipses

Rob Hawley

When a moon gets between its planet and the sun then the moon casts a shadow on the planet. This is no different from holding your hand in front of you and noticing the shadow on the ground. This shadow is rather complex, but this is discussed below.

Eclipses on the earth were once the subject of shamans and superstition. Even today in many countries well meaning local press put fear into their citizens that sunlight during an eclipse is somehow dangerous. (The sunlight itself is dimmer and thus less dangerous, but the extreme stories try to discourage people from looking at the sun).

Eclipses happen wherever moons cross in front of the sun. The only other planet that has easily observable eclipses is Jupiter. Jupiter’s moons regularly cross between the planet and the sun. Sky and Telescope publishes tables of these eclipses each month. Google offers a tool to make predictions on your  PC.

Eclipse Months

However, we are concerned with eclipses on the Earth. Eclipses on the earth are so spectacular because the moon and sun have about the same apparent size in our skies. This allows the moon to mask the brightest parts of the sun while not masking the corona.

There are two “Eclipse Months” each year. During each of these lunations either or both a solar eclipse (when the moon is between the sun and earth) or a lunar eclipse (when the earth is in between) can occur. This variability is due to the way the moon orbits the earth. The plane of the moon’s orbit is tilted by about 5° relative to the plane of the earth’s around the sun. The orientation of the plane is fixed. 12 lunations is not exactly a year so the plane drifts relative to the earth. We all know that because the calendar date of the new moon varies from year to year.

Eclipses can happen when the plane of the moon’s orbit crosses the plane of the earth’s. This happens 24 times each year. However, in only two months of the year is there a chance that the earth, sun, and moon will properly align. At other times the crossing is not oriented correctly. The figure at the left shows how the orbits of the moon and sun intersect during totality. This movie animates the drawing.

In 2008 two solar eclipses will occur (Feb 7 and Aug 1) and two lunar eclipses (Feb 21 and Aug 16). Not every year has all four events. A movie of a solar eclipse from my web site animates the picture on the left.  It slows to show the Feb 7 and August 1 eclipses.  At other new moons the moon is above or below the plane of the sun.

Even during these “Eclipse Months” there are no guarantees. One of the caveats is that the orbit of the moon is an ellipse not a circle. Thus the moon is sometimes closer and sometimes further away. If it is just slightly further away the moon will not completely cover the brightest parts of the sun. This causes an “annular” eclipse.

Returning to my hand analogy. If you hold your hand further from the ground you will notice the edges are indistinct. Since the sun is not a point source of light, one edge of the sun is illuminating areas that are hidden by the other. Similarly only a narrow band on the earth is fully in the shadow of the moon. In the rest some portion of the sun remains visible. These areas experience a partial eclipse.

The track of the shadow over the earth varies greatly. Since the plane of the moon’s orbit varies relative to the tilt of the earth, and the direction to the sun, the path of totality varies. The path repeats in regular cycles discussed here .

August 1 Eclipse

The next eclipse will occur this summer starting in the north polar regions and then passing almost due south through Siberia and then China.

A community of observers regularly travels to eclipses. I joined that community in 1999 in the Black Sea. The serious observers this year are going to Western China. I will be observing from Siberia (never been there, going to China next year). Some are even going to try to catch the eclipse by plane over the north pole!

Of course, weather may be a problem in Siberia. Oh Well I hear they have good vodka (and mosquitoes the size of birds)!

Web Exclusive - Here is a movie of the eclipse as seen from space.  Note this is a 13 Meg File and is best displayed full screen.  You will note a small dot starting almost at the north pole and then travelling south.  This is the area of totality.  As the dot moves south the earth is turning.  Thus the dot tracks a curve on the earth as shown at the left.

Coming Soon

China July 2009

Next year the moons shadow will again visit China. This time it will pass nearly west to east entering the ocean near Shanghai. Interestingly Iwo Jima is near the centerline.

South Pacific July 2010

The following year the track will cross the South Pacific. Ships will likely visit Pitcairn Island again as they did in 2005. Easter Island will also be under the track. Tahiti will be near the track.

Next US Visible Eclipse

Don’t want to travel to exotic locations to see one of these? In 9 years the track will be accessible by car.  The track will enter the US in Oregon, pass through Idaho and Colorado into the Midwest.

Just want to stay at home? You are going to have a wait! The next total eclipse visible from San Jose is in 2252.

Visiting an Eclipse

At this point it is likely too late to book a trip to the August eclipse. As I mentioned earlier, eclipses attract a crowd of regulars. Trips tend to book quickly. This would be a good time to think about 2009 and 10. While you can go on your own to Shanghai in 09, the logistics will be easier if you go with a group. The two companies I recommend are TravelQuest International and MWT Associates. Others are in the business, but I can personally vouch for these two.

Previous | Contents | Next