5The changing phase of the Moon… The un-changing face of the Moon… EclipsesLunar EclipsesSolar Eclipses
6Solar and Lunar Eclipses The total or partial obscuring of one celestial body by another…The obscuration can be eitherOne 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
7Solar EclipsesThe solar corona is revealed during a total solar eclipseThe 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.
8What 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 illuminatedUmbra is completely darkClick on the image to start animation
9Lunar eclipsesLunar eclipses happens when the Moon passes through the shadow of the EarthEverybody 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.Use the interactive figure to show the conditions for the 3 types of lunar eclipse.Click on the image to start animation
10Solar eclipsesSolar eclipses occur when the shadow of the Moon falls on the surface of EarthOnly 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 animationUse the interactive figure to show the conditions for the 3 types of solar eclipse.
11Eclipse 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
12Total Solar EclipseWhat’s the difference between a total and ring eclipse?The distance between the Earth and the Sun.You don’t see the Sun at all if you are in the umbraSurface of the EarthSunMoonYou see the partial Sun if you are in the penumbraClick on the Sun to start animationYou see the whole Sun outside of the shadow
13Ring EclipseRing eclipse happens when the Moon does not completely block the Sun, like partial eclipses…The umbra is NOT completely dark!You see a ring if you are in hereSurface of the EarthSunMoonYou see the partial Sun if you are in the penumbraClick on the Sun to start animationYou see the whole Sun if you are outside of the shadow
14Distance 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 ~ 0.016The distance between the Moon and Earth change. This is why we have total and ring eclipsesEccentricity of Lunar orbit is ~ 0.05
15Solar 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?
16How 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
17How 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 planeOnly when the nodes are located right between the Earth-Sun line can solar eclipses occur.
18The 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
19How Often Does Eclipse Occur? The combination of these motions of the Moon29 ½ day orbital motion around Earth,5º tilt of the orbit w.r.t. the ecliptic, andPrecession of the lunar orbit w.r.t. the ecliptic,Eclipse occurs with a period of about 18 years and 11-1/3 days Soras cycleBut these eclipses don’t happen at exactly the same location over the Soras cycle…
20Solar Eclipse Forecast Solar eclipses from 2004 to 2030Knowing 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
21Eclipses: Summary The parties involved: Sun, Moon, and Earth Motion of the Moon around Earth:29 ½ day revolution of the Moon around the SunTilt (~5º) of the lunar orbit (around the Earth) w.r.t. the Ecliptic plane (Earth’s orbital plane around the Sun)The precession of the lunar orbit w.r.t. Earth-Sun directionSolar 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.