What is an Eclipse?

An eclipse is when the light of an object is blocked by a closer one.  It’s most commonly used in astronomy to describe times when the Sun or Moon are shadowed.

Lunar eclipses occur the least of the two but are viewed more often; it's when the Earth passes directly between the Moon and the Sun and Earth’s shadow covers the Moon. Solar eclipses happen when the Moon crosses between the Sun and the Earth; Earth goes dark because of the Moon’s shadow.

Solar eclipses thus only occur during a New Moon, and because of the angle of the Moon’s ecliptic plane there are two to five a month.  And on top of that, only from certain regions on Earth can a solar eclipse be seen. So while they may not be a rare occurrence, they are a rare sight. 

1999 Solar Eclipse in FranceCredit: http://en.wikipedia.org/wiki/File:Solar_eclipse_1999_4_NR.jpg

Types of Solar Eclipses

  • Total eclipse – when the Moon is close enough to completely obscure the brightness of the sun, allowing you to see the solar corona.  During any such eclipse, totality occurs at best only in a narrow tract on the surface of the Sun that’s determined by the movements of the Moon and Earth. While total eclipses occur about every 18 months, they can only be seen from the same place roughly every 385 years.
  • Annular eclipse – when the Moon and Sun are in line, but the Moon isn’t close enough to block out all of the bright light. Instead, the Sun appears as an annulus, a bright ring, around the Moon’s outline.
  • Hybrid eclipse – when where you stand on Earth affects what you see.  Also known as an annular/total eclipse, a hybrid eclipse appears as a total eclipse at some locations and an annular eclipse at others. These are pretty rare.
  • Partial eclipse – when the Sun and Moon aren’t exactly in line, so the Moon only partially blocks the Sun. This can usually be seen a fair distance from the annular or total eclipse shadow track. Some eclipses are only partial eclipses, because the shadow track they form doesn’t land on Earth.

Viewing an Eclipse

The shadow the Moon makes travels because the Earth and Moon are moving through space. This is also what determines where on Earth an eclipse can be seen from. There are two parts to the shadow: the umbra and the penumbra. The umbra is the true dark shadow, the track that a total or annular eclipse can be seen from. The penumbra is the light shadow, from which a partial eclipse can be seen.

When looking at an eclipse, you have to prepare for it before it starts. You can’t just look up hoping to see it, because looking directly at the sun – even for a few minutes – can blind you. Plus, as the retina of your eye has no sensitivity to pain, you won’t even know you’re going blind.

The only time, even during an eclipse, that it’s safe to look at the sun is during the totality.  But that’s the shortest part of the event. Witnessing the full thing, the Moon passing in front of the Sun and then moving on, requires precautions.  Plus, the eclipse you may be witnessing might not be a total eclipse. This means using eye protection, or indirectly viewing the event.

 Viewing ToolsEclipser Solar Viewing GlassesCredit: www.amazon.com

  • Eclipse glasses – use a special solar filter in the lenses. Regular sunglasses won't cut it, nor will many homemade varieties.
  •  Indirect methods – involve projecting the image of the Sun onto another surface, usually a white sheet of paper. Pinhole camera made with binoculars or telescopes would work. It’s also possible to record the eclipse and then watch it on a screen, not through the optical viewfinder. (Though direct exposure to the Sun may damage the camera).

Just before the totality in a total eclipse, an effect called Baily’s beads will happen.  The light ring will appear bumpy, a result of sunlight reaching Earth through valleys on the Moon. A bright flash will follow, like that off of a diamond, when the moon's position fully blocks the sunlight. This signals the start of the totality.

At this point, you can look up at the sun without fear of damaging your eyes. The umbra moves at over 1700km/hr and so it never sits in one place for more than a few minutes. Enjoy the ability to look as the Sun for as long as you can, but then switch back to your glasses/pinhole camera/video screen when you see a second diamond flash, originating from the opposite side of the Moon as the first one. The eclipse will then go through the same stages as before, only in reverse.

Curious as to when the next eclipse is?  NASA has a site dedicated to solar eclipses. Good luck enjoying it!