C J L MARR

Solar Eclipses

an article written in 2009

 

If you lived on Mars, the word “eclipse” probably wouldn’t appear in your dictionary.  The moons Phobos and Deimos are so small that if they did pass in front of the Sun, it would hardly be noticeable.  The same would be true on Mercury or Venus where there are no moons at all.  In fact, solar eclipses like we see them on Earth, where the Moon passes between the Earth and the Sun, could be unique.

As explained in the article on angular sizes in the June-July 2008 edition of the Sidereal Times, the reason for our superb solar eclipses here on Earth is that the Sun is about 400 times larger than the Moon, and is almost 400 times further away than the Moon.  This stroke of exceptional luck (?) means that they have almost equal angular sizes (0.5 of a degree) and the Moon can exactly cover the bright disk of the Sun giving us total eclipses the like of which might not be naturally reproduced anywhere else in the universe!

During a total eclipse (pictured above courtesy Luc Viatour), the Sun’s bright disc is covered but the chromosphere (the bright gases just above the surface) and the corona (the faint outer atmosphere) are still visible.  If we’re really lucky we might glimpse an eruption, known as a prominence, from the surface of the Sun into the chromosphere.  Normally, of course, the intense light from the Sun’s photosphere (the gases at the surface) would prevent us seeing such wonders.

Sometimes, watchers can witness the “diamond ring” effect at the end of a total eclipse .  This is where the bright light from the Sun bursts out in one spot while the rest is still eclipsed.  This only happens when there is a valley on the Moon in the right position, so light can pass through it while the surrounding hills continue to block the light.  This effect only lasts a second or two, but I’m told it is well worth seeing.

The shadow created during a solar eclipse sweeps across the Earth at over 1,600 kph, so if you’re in exactly the right spot, the eclipse could last as long as 7.5 minutes.

Conditions have to be exactly right to create a total eclipse.  The Moon’s orbit around the Earth is slightly elliptical, and the Earth’s orbit around the Sun is the same, so there are times – for example, when the Moon is at its furthest point from Earth, but Earth is at its closest point to the Sun — when the Moon’s angular size is smaller than the Sun’s, and the Moon does not completely block the bright disk of the Sun.  When this happens, and there is a narrow ring of photosphere visible around the Moon, it is called an annular eclipse.  This is much less spectacular because the corona and chromosphere are much dimmer.

Then there are times when the Moon is not completely in line with the Sun, and we only see a partial eclipse.  During a  total  eclipse, only those lucky  folk in exactly the right area will witness totality.  For everybody else who catches a glimpse of the eclipse, it will only be partial. 

Eclipses of the Sun can only occur when the Moon is in its new Moon phase, because only then is it between the Earth and the Sun.  However, not every new Moon is accompanied by an eclipse.  In fact, eclipses are rare and this is because the Moon’s orbit is slightly inclined.  Therefore it is only when new Moon occurs while the Moon is in line with the Earth-Sun plane that an eclipse is possible.  If this was not the case, then there would be eclipses every 28 days, at every new moon.  Under those circumstances, they probably would not generate very much excitement.  However, the rarity of these wondrous events means that they are followed avidly, and people travel around the world to catch glimpses of them.