Solar Eclipse Size of Sun, Earth, etc.

1. Solar EclipseSize of Sun, Earth, etc. Lecture 6

2. Solar Eclipse is much harder to see than lunar eclipse Because of the different sizes of Earth umbra and lunar umbra : Moon is about 4 times smaller than Earth

3. Solar Eclipse as seen from the Moon 1999 Aug 11 solar eclipse Umbra was too small to seen in this scale. In the penumbra, people can see a partial solar eclipse.

4. Solar Corona can be seen during the total solar eclipse Tenuous outer part of the Sun’s atmosphere : several millions of degrees Can only be seen after blocking the visible disk of the Sun  blocking device is called “coronagraph”  Also useful in exoplanet imaging or imaging disks around other stars.  Image of a dusty disk in orbit around a nearby star

5. SOHO movie of grazing comets and coronal mass ejection

6. SOHO : 4 planets and Pleiades

7. Annular Solar Eclipse • When the Moon is at its farthest position from Earth (apogee), the Moon appears to be too small to cover the entire Sun  Annular Solar Eclipse • When the Moon is at its closest position from Earth (perigee), the width of total eclipse path can 270km wide.

8. Paths for total solar eclipses, 1997-2020

9. Predicting solar eclipses Prediction of solar eclipses was important to religious and political leaders. Thales is said to have predicted the eclipse of 585BC. • We know that solar eclipses can happen only at New Moon and the Sun has to sit on the line of nodes. • Therefore, any solar eclipses should be separated by multiple of synodic months. • The time required for the Sun to come back to the line of nodes is shorter than a solar year, and it is known as eclipse year (346.6 days). This is due to the precession of the Moon’s orbit. So, any solar eclipses need to be separated by multiple of eclipse year. • One can see that the smallest time interval simultaneously satisfies these two conditions is 6585 days ( = 223 x 29.53 days = 19 x 346.6 days). This interval is known as saros. From a more precise calculation, 1 saros is 6585.3 days (18 years 11.3 days). Due to the fractional days (0.3 day), total eclipse does not happen at the same geographical location.

10. Measuring the size of Earth Well before Copernicus, Greeks knew that the Earth is spherical shape by looking at the shadow during lunar eclipse. Around 200BC, Eratosthenes measured the size of the Earth. At Noon on Summer Solstice, measured the altitudes of the Sun at two places. Estimated circumference of the Earth was 250,000 stades which is about 42,000 km.

11. Syene and Alexandria need to be aligned North-South… I.e., the Sun needs to be transiting simultaneously at these two places.

12. Distance to the Sun • In 280BC, Aristarchus from the school of Alexandria measured the distance to the Sun compared to the Earth-Moon distance. • Measured angle was 87  Earth-Sun distance = 20 x Earth-Moon distance

13. Other sizes and distances… • By measuring the duration of the total lunar eclipse, Aristarchus estimated that the Moon is about 3 times smaller than the Earth. • Noting that apparent sizes of the Sun and Moon are the same, he concluded that the Sun has to be X times larger than the Moon (X is the ratio b/w the Earth-Sun distance and Earth-Moon distance).

14. Key Ideas from Chapter 3 • Lunar Phases: The phases of the Moon occur because light from the Moon is actually reflected sunlight. As the relative positions of the Earth, the Moon, and the Sun change, we see more or less of the illuminated half of the Moon. • Length of the Month: Two types of months are used in describing the motion of the Moon. • With respect to the stars, the Moon completes one orbit around the Earth in a sidereal month, averaging 27.32 days. • The Moon completes one cycle of phases (one orbit around the Earth with respect to the Sun) in a synodic month, averaging 29.53 days. • The Moon’s Orbit: The plane of the Moon’s orbit is tilted by about 5° from the plane of the Earth’s orbit, or ecliptic.

15. Key Ideas • The line of nodes is the line where the planes of the Moon’s orbit and the Earth’s orbit intersect. The gravitational pull of the Sun gradually shifts the orientation of the line of nodes with respect to the stars. • Conditions for Eclipses: During a lunar eclipse, the Moon passes through the Earth’s shadow. During a solar eclipse, the Earth passes through the Moon’s shadow. • Lunar eclipses occur at full moon, while solar eclipses occur at new moon. • Either type of eclipse can occur only when the Sun and Moon are both on or very near the line of nodes. If this condition is not met, the Earth’s shadow cannot fall on the Moon and the Moon’s shadow cannot fall on the Earth.

16. Key Ideas • Umbra and Penumbra: The shadow of an object has two parts: the umbra, within which the light source is completely blocked, and the penumbra, where the light source is only partially blocked. • Lunar Eclipses: Depending on the relative positions of the Sun, Moon, and Earth, lunar eclipses may be total (the Moon passes completely into the Earth’s umbra), partial (only part of the Moon passes into the Earth’s umbra), or penumbral (the Moon passes only into the Earth’s penumbra).

17. Key Ideas • Solar Eclipses: Solar eclipses may be total, partial, or annular. • During a total solar eclipse, the Moon’s umbra traces out an eclipse path over the Earth’s surface as the Earth rotates. Observers outside the eclipse path but within the penumbra see only a partial solar eclipse. • During an annular eclipse, the umbra falls short of the Earth, and the outer edge of the Sun’s disk is visible around the Moon at mid eclipse. • The Moon and Ancient Astronomers: Ancient astronomers such as Aristarchus and Eratosthenes made great progress in determining the sizes and relative distances of the Earth, the Moon, and the Sun.

18. In summary… Important Concepts Important Terms Corona Coronagraph Apogee & perigee Eclipse year Saros • Measuring the size of Earth • Measuring the size of the Moon • Measuring the size of the Sun • Measuring the distance to the Sun • Chapter/sections covered in this lecture : sections 3-5 and 3-6