Solar Eclipses

One of the most remarkable celestial phenomena visible from any part of the solar system is the total solar eclipse visible from Earth. This phenomenon is unique because of the nearly identical apparent sizes of the disks of the moon and the sun. When the moon passes in front of the sun it covers the photosphere, revealing a tremendous display of the extended, faint, solar corona.

Why is the total solar eclipse so unique? The other terrestrial planets have no significant satellites to mask the sun. The Jovian planets do have large satellites but, at their distances from the sun, the sun’s apparent size is much smaller than that of the satellites so that the eclipses must appear more like an occultation. Equally remarkably, the tidal interaction of the moon and the Earth causes the moon to gradually move outward in its orbit; eventually, the moon’s apparent size on the sky will shrink to the point that the moon’s shadow will not reach the Earth and total solar eclipses will no longer occur. This is still millions of years away.

Image not to scale.

The natural question is why doesn’t every new moon bring a solar eclipse? The moon’s orbit is tilted with respect to the ecliptic by 5 degrees so that at most new moons the moon’s shadow passes above or below the Earth. However, because of this tilt, the moon must pass through the ecliptic plane about twice a year. The point where the lunar orbit passes through the ecliptic is called a node and the line connecting the two nodes is called the line of nodes. If the moon is near a node during new moon, a solar eclipse will occur. The moon is close enough to a node for its shadow to fall on the Earth for about 38 days. This interval is called the eclipse season. Since the synodic lunar month, 29.5 days, is shorter than an eclipse season, a solar eclipse must occur during each eclipse season. In fact, if an eclipse occurs just at the beginning of the eclipse season, a second eclipse will occur at the end. In this case, both eclipses will probably be partial, the central part of the shadow (umbra) missing the Earth above and below each pole. In general, the eclipse is a total eclipse only about a third of the time; more often it is a partial or annular eclipse.

If the orbit of the moon were fixed with respect to the ecliptic, then total solar eclipses would occur twice a year within two weeks of some fixed date. However, because of gravitational tidal interactions with the Earth, all aspects of the moon’s orbit change with time.

First, the orbit itself revolves around the Earth. This is seen as the nodes moving backwards on the sky and, thus, the time the moon takes to pass the same orbital node twice, the nodical (or draconic -- for the dragon that swallows the Sun) month, is 27.2 days. As a result, the time between eclipse seasons is 9.3 days less than six months. So if the first eclipse season of the year occurs in early January, there is time for a third eclipse season in December. Second, the perigee time shifts around the orbit. The time between perigee passages, the anomalistic month, is 27.5 days.

Patterns do emerge but they take much longer than you would expect. The table shows the best known pattern, the saros cycle. Although quite long in comparison to the memory of a single observer, the saros cycle was discovered by numerous civilizations in the past. Most notably, the makers of Stonehenge included a saros calculation capability in their later design. The table shows that every 18 years plus 10 to 12 days (depending on leap years), a similar solar eclipse will occur. The extra third of a day means that the eclipse occurs a third of the world away. The first eclipse in a saros series is just a partial eclipse visible from one of the poles. Each subsequent eclipse in the series passes closer to the equator until, after about 71 eclipses spanning about 1300 years, the last eclipse in the series just misses the other pole.

Phases

There are four times of contact for the total solar eclipse. As you can see from the animation, most of the time is spent in the partial phases. First contact is defined as the first touch of the Moon over the Sun's disk. Then, for the next hour or so, the moon progressively covers the solar disk. At second contact, the Moon just covers the solar disk and totality starts. Baily's Beads, bits of solar surface visible through the valleys of the Moon, are visible just before the sun is totally covered. At the end of totality, which is always less than eight minutes and typically about three minutes, Baily's Beads reappear, signalling third contact. As you might expect, fourth contact occurs as the Moon's disk just totally uncovers the solar disk.

If the Moon's shadow reaches the Earth, the eclipse goes starts at sunrise at some location and the shadow races eastward at about 1000 km/hour until it reaches sunset. The path of totality is quite narrow, typically about 100 miles wide. A much larger part of the Earth can see the partial phases but they miss the exciting total phase. The figure below shows the paths of future eclipses for the next twenty-five years.

Solar eclipses are one of the most dramatic events in nature but, if you wait for one to come to you, the average delay will be about 300 years. Unlike many other dramatic natural phenomena (earthquakes, comets, volcanoes, etc.), solar eclipses are always predictable to the second and to the exact location. Better to wait only until one occurs at a location you’d like to visit and journey to join others in the shadow of the moon. Of course, if you're prepared, it will be safe and much more enjoyable.

Safety First
The most important fact to remember when observing a solar eclipse is that during partial phases, the sun is as dangerous to your eyes as it is when it is not eclipsed. The resolution elements of your eye, rods and cones, are much smaller than the image of the Sun projected on your retina. As a result, even when only a few percent of the Sun remains, the rods that see that sliver are getting all the light that they would receive if the disk were fully illuminated. Therefore, observe all the precautions that you would if it were an ordinary day. Don't look at the Sun with unprotected eyes for an extended period of time and never use any type of magnification (binoculars or telephoto lenses).

Typically, inexpensive Mylar glasses made of cardboard are available during eclipse times. They are safe to use if no magnification is used in front of them. If you want to use a telescope of binoculars, the solar filter must be the first optical element in the light train.

Between First and Second Contact
The sky will slowly become darker. During the first hour or so, this will seem just like a slightly cloudy day but, as totality draws closer, it will seem more like early twilight. A couple minutes before totality, if you're at a good site, you'll be able to see the shadow rushing at you from the west. Wise observers keep one eye covered during this time to give one eye the time to become dark adapted. The light of the corona is 1/500,000 the brightness of full sunlight ...not much brighter than full moonlight.

On horizontal surfaces, low-contrast dark bands known as shadow bands are often seen, becoming more organized and frequent as totality approaches.

Totality
The start and end of totality are charmingly denoted by Baily's beads or the diamond ring. Light from the Sun's surface shines through the valleys on the limb of the Moon, providing a moment of bright spots before the corona appears. During totality, you will be surrounded by sunset (in all directions!) and bright stars and planets will be visible. During the February, 1998 eclipse, Mercury, normally only seen as a faint planet just after sunset or just before sunrise, shone as brightly as Jupiter. High in the sky, it didn't have to shine all the way through many layers of atmosphere, haze, pollution, etc.

It is difficult to resist taking a picture or two during totality. However, unless you're an outstanding photographer with a clock drive mount and lots of experience, your photos will not look as nice as those taken by the serious eclipse photographers and, more to the point, not nearly as nice as what you can see with your unaided eye. So be sure to allot at least half your time to visual observing. With you dark adapted eye, you should see spectacular coronal streamers extending several solar radii. The large change in brightness from the inner to the outer corona makes it very difficult for photography to capture this effect but you eyes, with or without binoculars, will do a fine job.

Between Third and Fourth Contact
Party.