Eclipses And Other Occultations

Occultation is the term use to describe when one object in Space, passes in front of another object and in doing so, obstructs our view of it.

We are familiar with occultations of the Sun and the Moon. We give these occultations a special name – eclipses – solar eclipse for the Sun and lunar eclipse for the Moon.

Lunar Eclipses

A lunar eclipse occurs when the Earth, Sun and Moon are aligned, with the Earth between the Sun and the Moon.

The Earth is three times larger in diameter than the Moon, so it easily blocks out the Sun’s light and casts a shadow on the Moon.

The Moon of course, does not produce light itself, only reflecting sunlight, so it looks dark during an eclipse.

In fact the degree of darkness does vary due to the condition of Earth’s atmosphere. If Earth’s atmosphere is clean, it will bend sunlight to some extent and this gives the Moon a slight illumination.

If however Earth’s atmosphere is dusty, as happens after major volcanic eruptions, it does not bend sunlight so much and a lunar eclipse can be a very dark one.

As the Moon orbits around the Earth once every month approximately, you might expect there to be a lunar eclipse each month, but this does not happen.

The reason is that the orbit of the Moon is inclined a few degrees to the orbit of the Earth. This ensures the Moon does not pass into the Earth’s shadow every month.

However, lunar eclipses are more often seen than solar eclipses because when they do occur, they can be seen from everywhere on Earth where the Moon can be seen at that time.

When a solar eclipses takes place, it can only be seen from a narrow strip of places on Earth. The path of totality tracks across the Earth’s surface, with a strip of partial totality on either side.

Solar Eclipses

WARNING… Never, ever look at the Sun through binoculars or a telescope, as you will certainly damage your sight. Observe an eclipse only by projecting the Sun’s image onto a sheet of card.

Solar eclipses happen when the Moon is aligned between the Sun and Earth, so that sunlight is blocked-out and a shadow is cast on the Earth.

This gives a remarkable effect here on Earth, as the Sun is obscured and then reappears some minutes later.

From a particular point on Earth, a solar eclipse is sometimes partial, and occasionally total, meaning that the Sun is completely covered by the Moon.

During a total eclipse, the Moon just about exactly covers the Sun. This is remarkable in itself.

The Moon covers the Sun in this way because the ratio of their diameters, happens to be very similar to the ratio of their distances from the Earth.

Putting it another way, the Sun and Moon have roughly the same angular size in the Earth’s sky.

This exact coverage gives rise to wonderful effects at the edge of the Sun during total eclipses such as the “diamond ring” and the “string of beads”.

It has also enabled astronomers to study phenomena very close to the Sun’s surface, such as solar prominences and the Sun’s corona.

Other Occultations

Occultations of other objects in the sky are always keenly observed and have been important to astronomers in the past.

Lunar occultations are when the Moon passes in front of a star or planet and this is interesting to observe. The Moon has no atmosphere, so stars suddenly disappear without any fading and then later, suddenly come back.

Double stars have been discovered through observing lunar occultations.

Sometimes, an star will “graze” the edge of the Moon, revealing details of hills and mountains on the Moon, as the star disappears and then reappears several times.

As recently as 1977, faint rings were discovered around the planet Uranus because a star flickered as it was occulted by Uranus.

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