What is an eclipse of the Sun? What causes eclipses and why?
How often do eclipses happen and when is the next eclipse of the Sun?
You'll learn the answers to these questions and more in MrEclipse's primer on solar eclipses.
Before we learn more about the eclipses of the Sun, we need to first talk about the Moon.
Phases of The Moon
The Moon is a cold, rocky body about 2,160 miles (3,476 km) in diameter. It has no light of its own but shines by sunlight reflected from its surface. The Moon orbits Earth about once every 29 and a half days. As it circles our planet, the changing position of the Moon with respect to the Sun causes our natural satellite to cycle through a series of phases:
- New Moon > New Crescent > First Quarter > Waxing Gibbous > Full Moon >
Waning Gibbous > Last Quarter > Old Crescent > New Moon (again)
The phase known as New Moon can not actually be seen because the illuminated side of the Moon is then pointed away from Earth.
The rest of the phases are familiar to all of us as the Moon cycles through them month after month.
Did you realize that the word month is derived from the Moon's 29.5 day period?
To many early civilizations, the Moon's monthly cycle was an important tool for measuring the passage of time.
In fact many calendars are synchronized to the phases of the Moon.
The Hebrew, Muslem and Chinese calendars are all lunar calendars.
The New Moon phase is uniquely recognized as the beginning of each calendar month just as it is the beginning on the Moon's monthly cycle.
When the Moon is New, it rises and sets with the Sun because it lies very close to the Sun in the sky.
Although we cannot see the Moon during New Moon phase, it has a very special significance with regard to eclipses.
The Moon's Two Shadows
An eclipse of the Sun (or solar eclipse) can only occur at New
Moon when the Moon passes between Earth and Sun.
If the Moon's shadow happens to fall upon Earth's surface at that time,
we see some portion of the Sun's disk covered or 'eclipsed' by the Moon.
Since New Moon occurs every 29 1/2 days, you might think that we should
have a solar eclipse about once a month.
Unfortunately, this doesn't happen because the Moon's orbit around Earth
is tilted 5 degrees to Earth's orbit around the Sun.
As a result, the Moon's shadow usually misses Earth as it passes above
or below our planet at New Moon.
At least twice a year, the geometry lines up just right so that some
part of the Moon's shadow falls on Earth's surface and an eclipse of the
Sun is seen from that region.
The Moon's shadow actually has two parts:
- The Moon's faint outer shadow.
- Partial solar eclipses are visible from within the penumbra shadow.
- The Moon's dark inner shadow.
- Total solar eclipses are visible from within the umbra shadow.
1. Penumbra
2. Umbra
When the Moon's penumbral shadow strikes Earth, we see a partial eclipse of the Sun from that region.
Partial eclipses are dangerous to look at because the un-eclipsed part of the Sun is still very bright.
You must use special filters or a home-made pinhole projector to safely watch a partial eclipse of the Sun.
What is the difference between a solar eclipse and a lunar eclipse?
A lunar eclipse is an eclipse of the Moon rather than the Sun.
It happens when the Moon passes through Earth's shadow.
This is only possible when the Moon is in the Full Moon phase.
