Eclipses
Solar eclipses occur when the Moon's shadow falls on a small portion of the Earth. This can only occur during the new Moon phase. Because the Moon is smaller than the Earth a solar eclipse can only be observed from the small portion of the Earth that falls in the Moon's shadow.
In a total solar eclipse, the Moon blocks the entire solar disk. In a partial solar eclipse, it only blocks part of the Sun. A total solar eclipse can appear only partial outside the path of totality.
Lunar eclipses occur when Earth's shadow falls on the Moon. This can only happen during the full Moon phase. A lunar eclipse can be observed from roughly half the Earth - the nighttime half.
In a total lunar eclipse the entire Moon is engulfed in Earth's shadow. During a partial lunar eclipse, Earth's shadow only darkens part of the Moon. A total lunar eclipse will go through partial phases before and after totality.
Both solar and lunar eclipses occur about 2 or 3 times a year. Because solar eclipses are seen from only a small part of the Earth, they seem less common than lunar eclipses from any given portion of Earth.
Observing Solar Eclipses
You may have heard that looking at the Sun during a solar eclipse is dangerous. Well it is. It is always dangerous to look directly at the Sun. It can blind you. However if we try to stare at the Sun the extreme brightness pains our eyes. We immediately look away. During a solar eclipse, the sun is less bright but no less dangerous. The danger is there, but the protection mechanism is gone.
There are many solar filters that claim to make it safe to observe a solar eclipse. Some are safe; some are not. If you are not sure about a particular filter, assume it is not safe.
Projecting the Sun's image is safe. Take two index cards. Poke a pinhole in one of them, and place the other in its shadow. Adjust the distance between the cards to get the Sun's image. Using a shoe box with a pinhole in one end will also work and make it a little easier to see the projected image. A lens can also project the Sun's image. A removable camera lens, a telescope eyepiece, or binoculars can also project the image. Practice your technique before the eclipse.
During a total solar eclipse the sky will darken. Stars become briefly visible. Without the Sun's warming rays, the temperature drops slightly and you may feel a bit chilly. As the eclipse ends, it warms to the normal daytime temperature. Stars wink out as the sky brightens again.
Observing Lunar Eclipses
Observing a lunar eclipse presents no potential danger. As the Moon first enters Earth's outer shadow, the penumbra, there is no noticeable difference. When the Moon enters the darker inner shadow, the umbra, a small shadow begins to appear on the Moon. In a partial lunar eclipse, the shadow will not cover the entire Moon. In a total lunar eclipse, this shadow gradually grows until it covers the entire Moon.
During this total phase, the moon will appear dark. It may be a dark gray, or it may be a dark ruddy coppery color. After totality is over, the Moon gradually leaves Earth's shadow and returns to the normal full Moon.
Enjoy the next eclipse.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
