Solar Eclipses

Solar Eclipse
Diagram showing alignment of the Sun, Earth and the Moon during a total solar eclipse (click to enlarge)

A solar eclipse occurs when the Moon passes between the Sun and an observer on the Earth‘s surface.

Since the Moon’s orbits plane is inclined by 5 degrees to that of ecliptic plane, (in which the Earth’s orbit path around the Sun lies) a solar eclipse does not occur at every new Moon.

Although solar eclipses occur more often than lunar eclipses, they are only visible to observers directly beneath the line traced by the Moon’s shadow over the Earth’s surface. Therefore, fewer people witness solar eclipses when they occur than lunar eclipses, which are observable anywhere on Earth where the Moon is visible.

The Sun’s corona visible during a total solar eclipse on 21 August 2017 above Madras, Oregon.
Photo Credit: (NASA/Aubrey Gemignani)

Total Solar Eclipses

A total solar eclipse occurs when the Moon passes directly over the Sun’s disc, blocking it out completely.

Due to the coincidence that the Moon appears almost exactly the same same size in the sky as the Sun to an observer on Earth, the Moon can block out the Sun’s disc entirely, while allowing the Sun’s faint corona to become visible around the circumference of the Moon.

The total phase of a solar eclipse is the only time it is safe to look at the Sun directly.

Bailey’s Beads and the Diamond Ring

Bailey’s Beads is the name given to the effect during the beginning and end of the total phase of a solar eclipse, when the Sun’s rays peak out between mountain peaks or other topographical features on the edge of the Moon’s disc. The explanation for this effect, likened to a string of beads, was first given by Francis Bailey in 1836.

The “diamond ring” effect is produced when only one of these beads is visible, giving the appearance of a bright diamond set in the ring of the Sun’s corona.

Note that it is NOT safe to view either Bailey’s Beads or the Diamond Ring effect without proper eye protection. See “Safely observing a Solar Eclipse” below.

Partial solar eclipse
A partial solar eclipse, with a cluster of sunspots visible near the Sun’s limb.
(Click to enlarge)

Partial Solar Eclipses

A partial solar eclipse appears to an observer on Earth when the Moon’s disc doesn’t line up entirely with that of the Sun. The observer of a partial solar eclipse is in the penumbra of the Moon’s shadow.

Both total and annular eclipses (see below) have a ‘partial phase’ before and after the Moon is completely aligned over the Sun’s disc.

Annular Eclipses

An annular eclipse occurs when the Moon is near to it’s apogee – the furthest point from the Earth in its elliptical orbit – making it appear slightly smaller in the sky than the Sun’s disc. Rather than blocking out the Sun completely, as in a total eclipse, a circular ring of fire from the Sun’s photosphere is visible around the Moon’s disc, known as the annulus.

Annular Solar Eclipse
Annular solar eclipse imaged by the Hinode satellite on 4 January 2011. Image Credit: NASA/Hinode/XRT via

During an annular eclipse, the observer is positioned in the ‘antumbra’ of the Moon’s shadow.

Safely observing a Solar Eclipse

Warning: you should never look directly at the Sun (except during the total phase of a solar eclipse), as this could result in permanent blindness or damage to the eyes.

The safest way to view a solar eclipse is by projecting  the Sun’s image through a pinhole projector. Welder’s glasses rated 14 or higher, or special eclipse eyeglasses can also be used. However, make sure that eclipse glasses are specifically designed for this purpose. Other types of translucent, coloured or smoked glass or film will not protect against the Sun’s UV rays and will result in permanent eye damage or blindness.

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Astronomy, Cosmology, Space and Astrophysics