Posts Tagged ‘Umbra’

Total vs Annular eclipses

27 May, 2009

While total solar eclipses are awesome, unfortunately, not every eclipse of the Sun is a total eclipse.

Sometimes, the Moon appears smaller than the Sun’s disc, which results in an annular eclipse, where the Sun’s apparent larger size creates a very bright ring, or annulus, around the silhouette of the Moon.

total vs annularThis happens because of the Moon’s orbit around the Earth — it is not perfectly round, but oval, or elliptical, in shape. As the Moon orbits Earth, its distance varies from around between 221,000 to 252,000 miles. (This 13% variation in the Moon’s distance from Earth also explains the Moon’s apparent size in the sky varies by the same amount.)

So, when the Moon is on the near side of its orbit, it appears larger to us than the Sun. If an eclipse happens at that time, it will be a total eclipse. In the umbra shadow, we would see the Moon completely block the Sun. Fig. (A)

However, if the eclipse happens when the Moon is on the far side of its orbit, the Moon will appear smaller than the Sun, and can’t completely cover it, giving us an annular eclipse. The name comes from the annulus, of the bright ring, around the outline of the Moon. On Earth, we would be located in the antumbra shadow. Fig. (B)

In both total and annular eclipses, places that fall in the path of the penumbra will see a partial eclipse. Fig. (C)

Check out these images comparing total eclipses to annular eclipses, where the apparent size of the Moon is the same as the Sun in one, blocking it totally, and smaller than the Sun in another, leaving a ring around the Moon’s silhouette.

01 AUG 2008 Solar_Eclipse_GansuRingfoermigeSonnenfinsternis