Eclipses

Eclipse: BrainPOP

• http://www.brainpop.com/science/space/eclipse/

Eclipse worksheet for BrainPOP

• http://www.middleschoolscience.com/BrainPOPeclipse.pdf

Eclipse worksheet

• http://www.middleschoolscience.com/eclipse.pdf

• The changing phase of the Moon…

• The un-changing face of the Moon…

• Eclipses– Lunar Eclipses

– Solar Eclipses

Solar and Lunar EclipsesEclipse:

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

The obscuration can be either• One celestial body blocking

the view to the other:– Solar eclipse---Moon

blocking Earth’s view to the Sun…

• One celestial body is in the shadow of another:– Lunar eclipse---Moon is in

the shadow of the Earth…

Lunar eclipse image from http://www.mreclipse.com

Solar Eclipses

The solar corona is revealed during a total solar eclipse• The corona is about one

millionth times fainter than the disk of the Sun.

• Similar to observing stars next to the Sun, the light from the disk must be blocked (by the moon, or by special occulter in the telescope) before we can see the solar corona.

What Causes Eclipse?• The Earth and Moon cast shadows.

• When either passes through the other’s shadow, we have an eclipse.

• Because the Sun is an extended bright object, there are two different regions of the shadow:

– Penumbra is partially illuminated

– Umbra is completely dark

Click on the image to start animation

Lunar eclipses

• Lunar eclipses happens when the Moon passes through the shadow of the Earth– Everybody on the night side

of Earth can see the lunar eclipse.

• Lunar eclipses can be partial, penumbral, or total.

• Lunar eclipses can occur only at full moon.

Click on the image to start animation

Solar eclipses

• Solar eclipses occur when the shadow of the Moon falls on the surface of Earth– Only people in the shadow

can see the eclipse.

• Solar eclipses can be partial, annular, or total.

• Solar eclipses can occur only at new moon.

Click on the image to start animation

Eclipse Path

When total or ring solar eclipse happens…• The diameter of the umbra of lunar shadow is no more than

270 km…you can see the total eclipse only if you are in a very narrow and long eclipse path.– In the 1990 total eclipse that passed through the big island of Hawaii,

people on Maui and Oahu cannot see the total eclipse!

• The diameter of the penumbra of the lunar shadow is about 7000 km (Earth’s diameter is about 13,000 km). So, the region that partial eclipse can be seen is quite large…and people on Maui and Oahu could see partial eclipse!

Eclipse Prediction

Total Solar EclipseWhat’s the difference between a total and ring eclipse?The distance between the Earth and the Sun.

You see the whole Sun outside of the shadow

You see the partial Sun if you are in the penumbra

You don’t see the Sun at all if you are in the umbra

Moon

Surface of the Earth

Sun

Click on the Sun to start animation

Ring Eclipse• Ring eclipse happens when the Moon does not

completely block the Sun, like partial eclipses…

You see the whole Sun if you are outside of the shadow

You see the partial Sun if you are in the penumbra

• The umbra is NOT completely dark!

• You see a ring if you are in here

Moon

Surface of the Earth

Sun

Click on the Sun to start animation

Distance Between the Moon and Earth

• Like the Earth’s orbit around the Sun, the orbit of the Moon around the Earth is not a perfect circle. – Eccentricity of Earth’s orbit is

~ 0.016

• The distance between the Moon and Earth change. This is why we have total and ring eclipses– Eccentricity of Lunar orbit is

~ 0.05

Solar Eclipses: A Coincidence?

• What if the Moon is closer (say twice as close) to the Earth?

• What if the Moon is farther (say twice as far) away from the Earth?

• What if the Moon is two-time bigger?

• What if the Moon is two-time smaller?

How Often Does Eclipse Occur?

• Do we have one solar and one lunar eclipse every month, since we have a full and a new Moon every month?

Click on the image to start animation

How Often Does Eclipse Occur?• Do we have one solar and one lunar eclipse every six month,

when the nodes line-up with the Earth-Moon line?– NO! Because of the precession of the lunar orbit!

Nodes: the two points when the Moon crosses the ecliptic plane• Only when the

nodes are located right between the Earth-Sun line can solar eclipses occur.

The Precession of Lunar Orbit

• The orbital motion of the Moon around Earth slowly precesses with an 18.6 year cycle as the Earth orbit the Sun…

Click on the image to start animation

How Often Does Eclipse Occur?

• The combination of these motions of the Moon1. 29 ½ day orbital motion around Earth,2. 5º tilt of the orbit w.r.t. the ecliptic, and3. Precession of the lunar orbit w.r.t. the ecliptic,

• Eclipse occurs with a period of about 18 years and 11-1/3 days Soras cycle

• But these eclipses don’t happen at exactly the same location over the Soras cycle…

Solar Eclipse ForecastSolar eclipses from 2004 to 2030

Knowing the orbit of the Earth and the Moon, we can now calculate the time and path of solar eclipses with great accuracy.

Back to Eclipse Path

Eclipses: Summary• The parties involved: Sun, Moon, and Earth• Motion of the Moon around Earth:

1. 29 ½ day revolution of the Moon around the Sun2. Tilt (~5º) of the lunar orbit (around the Earth) w.r.t. the Ecliptic plane (Earth’s

orbital plane around the Sun)3. The precession of the lunar orbit w.r.t. Earth-Sun direction

• Solar eclipse happens when the Moon is between the Earth and the Sun.• The size and distance of the Moon need to be just right for us to see total

eclipse.– The changing distance between the Earth and the Moon explains the

occurrences of the total and ring solar eclipses. – The changing distance between the Earth and the Sun, and the Earth and the

Moon explains the difference in the duration of the solar eclipses. • Lunar eclipse happens when Earth is between the Moon and the Sun.