Eclipses

What is an eclipse?

The total or partial obscuring of one celestial body by another.

There are two types of eclipses that we observe from the Earth – Lunar eclipses and Solar eclipses.

Reviewing Celestial Motions

The Earth revolves around the Sun – 365 days.

The Moon revolves around the Earth just about along the plane of the Equator – so the orbital plane of the Moon is tilted around 5 degrees from the orbital plane of the Earth around the Sun. (Ecliptic)

Solar Eclipse

• A SOLAR ECLIPSE occurs when the Moon moves between the Earth and the Sun and blocks some or all of the Sun’s light.

• Due to the 5 degree tilt of the Moon’s orbit, the concept of aphelion / perihelion and apogee / perigee, the types and frequency of Solar Eclipses are somewhat complex to understand.

Total Solar Eclipse

Total Solar Eclipse occurs when the Moon is near perigee and the Earth is near aphelion. Because the Moon is closer it appears a bit larger in the sky and because the Sun is further it appears a bit smaller. In this way, the Moon blocks most of the Sun. In fact, only the outermost layer of the Sun, its CORONA, can be seen during a total solar eclipse

Annular Solar EclipseIf the Moon is near apogee and/or the Earth is near perihelion, the Moon will not appear big enough to completely block out the light of the Sun. This eclipse is called an ANNULAR ECLIPSE. Much of the sunlight still gets through to the Earth, and therefore the annular eclipse is still dangerous to look at directly.

Partial Solar Eclipse

If conditions are such that the Moon does not completely obscure the Sun, a partial solar eclipse is seen. Partial eclipses are also seen from certain areas on the Earth during a total eclipse (We’ll talk more about this in a moment.)

A little geometry…

Solar eclipses typically last a little more than 7 minutes. Because the Earth is rotating and the Moon is revolving around us, the “path” of the eclipse moves across the Earth. The path where the total eclipse can be seen is very narrow – about 100 miles wide. Outside of this path is a much wider area in which a partial eclipse is seen.

Umbra / Penumbra

People on the Earth who are inside the main shadow of the Moon, or UMBRA, see a total solar eclipse. People outside of the umbra but in the PENUMBRA see a partial eclipse.

The Sun, blocked by the Moon, casts a shadow on the Earth. There are two types of shadow which are cast due to the fact that the Sun is so much larger than the Moon.

Lunar Eclipses

When the Earth ends up between the Sun and Moon, it blocks the Sun’s light from hitting the Moon. This event is called a LUNAR eclipse.

Like Solar eclipses, Lunar eclipses can be full or partial depending on where the observer is.

The principle of the UMBRA and PENUMBRA apply to the Lunar eclipse also because the Earth is much smaller than the Sun. In the case of the Moon, it will first be partially obscured by the Earth and a round shadow will be seen blocking part of the Moon…

Once the Moon completely moves into the umbra, it is darkened, however – light from the Sun strikes the Earth’s atmosphere and gets bent or REFRACTED towards the Moon. The gases and dust in the atmosphere filter the light to a reddish color and so during the total Lunar eclipse the Moon appears orange or red.

Eclipse Frequency

• Solar eclipses generally happen twice a year, but can occur up to seven times a year if conditions are right. HOWEVER, a total solar eclipse only is visible from the same place on the Earth every 370 years. The next total solar eclipse visible from the United States will occur in August, 2017.

• Lunar eclipses also occur at least twice per year.

History