When people talk about eclipses they are referring to one of two occurrences; Lunar Eclipses and Solar Eclipses. Both of these occur when the Earth and the Moon are oriented in a certain way in relation to the Sun. Eclipses, like any astronomical event are worth noting on your calendar not only because of they are spectacular in their own right, but because they offer a new and exciting spectacle different from night to night observing.
Lunar eclipses occur when the Earth is positioned exactly in front of the Moon, blocking the Sun’s light from reaching the Moon’s surface. Because the Moon must be behind the Earth, lunar eclipses only occur during full moons. They do not occur every full moon however, because the orbit of the Earth around the Sun, and the orbit of the Moon around the Earth are not exactly aligned. As a result lunar eclipses occur every six months or so. Depending on how much of the Earth’s shadow is cast on the Moon determines whether it is a total or partial eclipse, as this varies from eclipse to eclipse.
The appearance of a Lunar eclipse also varies depending on a number of factors. The shadow cast on the Moon can differ in color from bluish white, if the moon passes through the edge of the Earth’s shadow, to deep orange coppery reds if the center of the shadow reaches the Moon’s surface. These colors appear because as light passes through the Earth’s atmosphere on the way to the Moon, it gets refracted or bent and as a result changes color as well. It is for this reason that the state of the Earth’s atmosphere will affect how a lunar eclipse appears. On days when the atmosphere is clear the most light makes it through producing bright and colorful eclipses, while dark and cloudy days often produce eclipses that are very dark. Some eclipses are so dark in fact that the can be hard to see at all as the Earth appears to simply black out the Moon.
Lunar eclipses are best viewed through binoculars, as they will bring out the colors more vibrantly than can be seen with the naked eye. Lunar eclipses last for about an hour or so, so there is plenty of time to view them during a stargazing outing. Additionally lunar eclipses are great because they can be seen anywhere in the world the Moon is visible at the time of the eclipse, unlike solar eclipses which require the observer to be in specific geographic location.
Solar eclipses are much harder to observe for several reasons than lunar eclipses but often worth seeing as most people agree are a much more spectacular event. Solar eclipses occur when the Moon transits across the Sun, or passes directly in front of it in the sky, casting a shadow onto the Earth. As it happens the Moon and the Sun appear roughly the same size in the sky and when then Moon is centered over the Sun it can completely envelop it leaving only a bright ring surrounding a dark sphere.
Unlike lunar eclipses, one’s location on the Earth will determine how the Solar eclipse appears. This is because the Moon, being smaller than the Earth, will only cast a shadow on a portion of the Earth, and depending on whether you happen to lie in the center of its shadow or not will determine whether the Moon’s path across the Sun will either only partially cover it, called a partial eclipse, or cover it entirely. The more sought after sight obviously is the Moon centered perfectly over the Sun, and people will often travel around the world to witness this occurrence.
Solar eclipses fall into two types depending on the apparent size of the Moon and the Sun in the sky. The Moon’s distance varies slightly from the Earth, as does the Earth’s distance from the Sun. This causes the apparent size of the Moon and Sun to vary slightly as well from eclipse to eclipse. When the Moon appears smaller than the Sun and can only cover a portion of its profile, leaving a bright ring around the dark circle of the Moon, this is called an annular eclipse. When the Moon’s profile is larger than the Sun’s and it is able to completely cover the Sun, leaving only its outer corona visible, this is called a total eclipse.
Similar viewing methods for viewing the Sun (click here) should be practiced when viewing solar eclipses. The only exception to this is during total solar eclipses during the brief period when the Moon entirely covers the Sun and none of its surface is visible. During this time it is safe to view with the naked eye and even binoculars, but it is important to wait until the last slivers of the Sun disappear from view and then to immediately discontinue observation when it emerges again, otherwise you risk damaging your eyes. In fact if you’re not comfortable doing this it is recommended that you stick to safer viewing methods.
Solar eclipses typically last for about an hour or two with the Moon partially covering the Sun for about an hour leading up to totality, when the Moon is perfectly centered over the Sun. Totality can last anywhere from a few seconds to a couple of minutes depending on the eclipse. Afterwards the Moon’s path drifts away from the Sun and the eclipse eventually ends.
During totality, while the Sun’s surface is hidden, the eclipse will appear to be a dark circle in the sky surrounded by a ring of silvery strands of light emerging from the sides. These lights are the corona of the Sun, or the “atmosphere” of plasma which makes its outer most layer. Also at this time if you happen to be in the shadow of the Moon you will notice how it momentarily becomes dark in your surroundings here on Earth. It won’t become as dark as night time, but it does leave an eerie feeling, unusual for the day time.
Solar eclipses occur, anywhere from 2 to 5 times a year, but often times they can only be seen as partial eclipses as the center of the Moon’s shadow actually passes over or underneath the Earth. Total eclipses occur about once every year and a half or so. Additionally it is unlikely that the central path of the Moon’s shadow will pass over where you live so if a total eclipse happens to occur over your backyard consider yourself lucky and don’t squander this amazing opportunity.