Eclipses - Understanding Shadows

Transcription

1 1 Eclipses - Understanding Shadows An eclipse occurs when one astronomical object casts a shadow on the other. Solar Eclipses The Sun casts a shadow on the Earth Lunar Eclipses The Earth casts a shadow on the Moon

2 2 The Geometry of Shadows A shadow created from an extended source of light (e.g. The Sun) has two parts. A dark umbra ( A ) in which all light from the Sun is blocked A less shaded penumbra ( B, C, D ) where part of the light from the Sun is blocked.

3 3 Moon Phase and Eclipses Because of the required alignment between Sun, Moon, and Earth eclipses either happen at Full or New Moon For a total solar eclipse the Moon is New. For a total lunar eclipse the Moon is Full. Not to Scale!!!!

4 4 Why Eclipses are Rare The Earth and Moon, when seen to true scale, are tiny compared to their separation. Alignment must be nearly perfect. The tilt of the Moon's orbit hinders that alignment. If this figure were true to scale the Moon would be twice as far from the Earth!!

5 5 Why Eclipses are Rare The Earth and Moon, when seen to true scale, are tiny compared to their separation. Alignment must be nearly perfect. The tilt of the Moon's orbit hinders that alignment.

6 6 Quantitatively How Limited are the Opportunities? If we were restricted to observing partial solar eclipses from the center of the Earth, the Moon could be only a little more than 1/2 degree off of the Ecliptic and it's disk would miss the disk of the Sun. The angular diameters of the Earth and Moon are both about 1/2 degree. There would be little tolerance for seeing total solar eclipses

7 7 Quantitatively How Limited are the Opportunities? If we were restricted to observing partial solar eclipses from the center of the Earth, the Moon could be only a little more than 1/2 degree off of the Ecliptic and it's disk would miss the disk of the Sun. The angular diameters of the Earth and Moon are both about 1/2 degree. There would be little tolerance for seeing total solar eclipses

8 8 Quantitatively How Limited are the Opportunities? If we were restricted to observing partial solar eclipses from the center of the Earth, the Moon could be only a little more than 1/2 degree off of the Ecliptic and it's disk would miss the disk of the Sun. The angular diameters of the Earth and Moon are both about 1/2 degree. There would be little tolerance for seeing total solar eclipses

9 9 Quantitatively How Limited are the Opportunities? If we were restricted to observing solar eclipses from the center of the Earth, the Moon could be only a little more than 1/2 degree off of the Ecliptic and it's disk would miss the disk of the Sun. However, the Earth is a large target and it's apparent angular radius as seen from the Moon adds about a degree to the target range expanding it to 1.5 degrees.

10 10 Quantitatively How Limited are the Opportunities? If we were restricted to observing solar eclipses from the center of the Earth, the Moon could be only a little more than 1/2 degree off of the Ecliptic and it's disk would miss the disk of the Sun. So how far around the 5-degree inclined orbit of the Moon do you have to go before the separation from the ecliptic reaches 1.5 degrees.

11 11 Why Eclipses are Rare The Earth and Moon, when seen to true scale, are tiny compared to their separation. Alignment must be nearly perfect. There are two times a year, separated by 6 months, when the shadows line up. This eclipse season drifts in the calender due to the precession of the Moon's tilted orbital plane with a period of 18.6 years.

12 12 Solar Eclipses The tapering umbral shadow of the Moon just barely reaches Earth (sometimes it doesn't an annular eclipse) The Moon's umbral shadow is small covering a couple of hundred miles at best.

13 13 Solar Eclipses The Moon's orbital motion (minus Earth rotation) sweeps the shadow across the Earth in a matter of hours. Locations along this eclipse path experience totality for a few minutes at best.

14 14 Solar Eclipses The Moon's orbital motion and Earth rotation sweep the shadow across the Earth in a matter of hours. Locations along this eclipse path experience totality for a few minutes at best.

15 15 Solar Eclipses Despite the short duration and remote location, people go to extremes to view eclipses. Phenomena that are usually washed out by the blue sky become visible the solar corona and solar prominences.

16 16

17 17

18 18

19 19

20 20

21 21 also

22 22 also

23 23 Note the nearly identical eclipses 18+ years apart. The Moon's exact position repeats relative to the Earth and Sun every 18 years creating a family of eclipses the Saros cycle.

24 24 Solar Eclipses from Charlottesville? You have to wait several hundred years on average for a total solar eclipse to happen at your location. Partial eclipses, which cover more area, are common however.

25 25 Eclipses in the Next Century

26 26 Eclipses on Other Worlds The outer planets have lots of moons...

27 27 Lunar Eclipses The Earth casts a shadow on the Moon To be specific the Moon moves through the Earth's shadow

28 28 Lunar Eclipses The Moon takes a couple of hours to cross the Earth's shadow. The Earth's umbral shadow is large enough to consume the whole Moon.

29 29 Lunar Eclipses Everybody on the night side of the Earth can see the eclipse.

30 30 Lunar Eclipses The Moon takes on a reddish hue during the total eclipse because of light refracted through the Earth's atmosphere. This is the combined light of all of the world's sunrises and sunsets! The View from the Moon

31 31

32 32 Annular vs. Total Eclipses The Moon follows an elliptical orbit. When it is close to the Earth it easily covers the Sun (total eclipse) when far away its angular size is smaller than the Sun's (annular eclipse).

33 33 Annular vs. Total Eclipses So much Sun is visible during an annular eclipse that none of the spectacular solar eclipse phenomena is visible.