Eclipses Explained

Solar Eclipses

Birds nest at midday, chirp night
songs in midday twilight—night
without sunset, the sun noon
high, bruised black by the moon.
— Deborah Trustman, "The Eclipse", 1975

Solar Eclipses A solar eclipse occurs when the new Moon passes between the Sun and the Earth, casting a shadow on the Earth. There are three types of solar eclipses: annular, partial, and total.

A partial solar eclipse occurs when the Sun and the Moon are not quite lined up. In this case, the Moon covers only a section of the Sun. This kind of eclipse usually goes unnoticed by most people on Earth because the sunlight is scarcely dimmed.

A total solar eclipse, however, is much more spectacular. At totality, the disc of the Sun is completely obscured by the new Moon, and only the ghostly solar corona and perhaps gigantic solar flares are visible around the edges of the Moon. Those parts of the Earth experiencing totality become very dark and it is possible to see stars in the sky.

An annular solar eclipse is similar to a total eclipse except that the new Moon is near apogee and its disc appears slightly smaller than that of the Sun. Thus, a bright ring or annulus of the solar disc remains at maximum eclipse, and the sky does not become dark.

There is actually a fourth kind of solar eclipse, an extremely rare hybrid solar eclipse which appears annular from some vantage points along the central eclipse path and total from other places on that path.

There are at least two and sometimes as many as five solar eclipses every year. Why don't we have a solar eclipse during every new Moon? This is because the Moon's orbit around the Earth is at an angle to the ecliptic. The new Moon usually passes above or below the Sun as seen from the Earth and thus there is no eclipse. The proper alignment for a solar eclipse occurs only a few times a year.

Lunar Eclipses

Thy shadow, Earth, from Pole to Central Sea,
Now steals along upon the Moon's meek shine
In even monochrome and curving line
Of imperturbable serenity.
— Thomas Hardy, "At a Lunar Eclipse", c 1865

Lunar Eclipses Another kind of eclipse is a lunar eclipse which occurs when the Moon passes through the shadow cast by the Earth. A lunar eclipse only occurs when the Moon is full and for the same reasons given above, happens only two or three times a year.

A lunar eclipse may be total, partial or penumbral. A total lunar eclipse causes the full Moon to slowly darken as it enters the umbra of the Earth and at totality, the Moon may take on a dark coppery colour. During a partial lunar eclipse, the Moon does not fully enter the umbra of the Earth. A penumbral lunar eclipse is likely to go unnoticed as the Moon passes through the Earth's penumbra and does not dim a great deal.

Observing Eclipses

You risk permanent damage to your eyes if you look at the Sun directly or through a camera viewfinder or through any kind of magnifying lens (for instance, binoculars or a telescope) unless you use proper solar filters. The safest way to observe a solar eclipse is to use a projection technique such as that employed by a pinhole camera.

If you wish to use binoculars, a camera or a telescope, then you must use a solar filter. Be sure that it is fitted on the Sun-side of the lens, not the eye-side. Cheap eyepiece filters such as those sold with inexpensive telescopes should never be used. Solar filters can be bought from camera and telescope dealers. Retinal damage can happen in seconds and cannot be repaired, so don't take chances.

Only the total phase of a solar eclipse can be viewed safely without filters.

The Moon is much less bright than the Sun so it is perfectly safe to observe a lunar eclipse without special protective equipment.

Your eyesight is precious. Protect it!