Solar Eclipses

Solar eclipses occurs when the Moon passes between the Earth and the Sun. This three-body alignment is known as a syzygy.

A little quirk of nature means that the diameter of the Sun is 400 times greater than that of the Moon’s, but the Sun is on average 400 times farther away from the Earth, so both appear to have the same angular diameter (0.5°) in the sky. It’s this combination that allows us to see solar eclipses.

When the Moon is directly between the Earth and the Sun, it casts a conical shadow (about 480 km across) on the Earth. The darkest part of the shadow is called the umbra and its narrow path is approximately 160 km (average) in diameter and no part of the Sun can be observed at totality from Earth. The lighter part of the shadow is known as the penumbra and the Moon will only partially cover the Sun. What we see will depend on where we are in the Moon’s shadow.

Standing at the point on the Earth where the umbra falls, results in observers seeing the disk of the Moon completely covering the disk of the Sun. This is called a Total Solar Eclipse. Observers only slightly removed from the umbra shadow would be in the penumbral shadow and see only part of the Sun’s disk covered by the Moon. This is called a Partial Solar Eclipse.

Occasionally at the time of an eclipse, the Moon is farther away from Earth, and therefore its apparent size is smaller and may not completely cover the Sun’s photosphere (a narrow rim of the Sun can be seen). This is called an Annular Eclipse. If the Moon’s orbit was exactly in the plane of the Earth’s orbit, then we would see an eclipse of the Sun every month. However, this is not the case with the Moon’s orbit inclined by 5°. Thus, solar eclipses occur two to five times a year, with one particular locality seeing a total eclipse about once every 350 years.

The Moon’s shadow sweeps across Earth at speeds of between 1,700-8,000 km/h (average 3,683 km/h) depending on where along the path you observe from, due to a combination of the Earth’s rotation and the Moon’s orbital motion, thus a solar eclipse occurs over a short period of time.

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CAUTION! It is dangerous to watch the eclipse while the Moon is moving across the Sun. Specialist eye and camera protection is essential. Make sure you don’t use photographic-only filters visually as these can still cause damage to your eyes. Eclipse glasses are an ideal (and safe) way to watch a solar eclipse however make sure they carry safety standard labelling (usually small print on the inside of the glasses) before using.

Once totality occurs it is safe to look directly at the eclipse without eye protection but as totality will be very short for the 2023 total solar eclipse (about 50-60 seconds depending on your location) observers must be extremely careful!.