Because of the different sizes of Earth umbra and lunar umbra :
Moon is about 4 times smaller than Earth Solar Eclipse is much
harder to see than lunar eclipse
Slide 3
1999 Aug 11 solar eclipse Umbra was too small to seen in this
scale. In the penumbra, people can see a partial solar eclipse.
Solar Eclipse as seen from the Moon
Slide 4
Tenuous outer part of the Suns 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. Solar Corona can be
seen during the total solar eclipse Image of a dusty disk in orbit
around a nearby star
Slide 5
SOHO movie of grazing comets and coronal mass ejection
Slide 6
SOHO : 4 planets and Pleiades
Slide 7
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 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.
When the Moon is at its closest position from Earth (perigee), the
width of total eclipse path can 270km wide. Annular Solar
Eclipse
Slide 8
Paths for total solar eclipses, 1997-2020
Slide 9
We know that solar eclipses can happen only at New Moon and the
Sun has to sit on the line of nodes. 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. 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 Moons orbit. So, any solar eclipses need to be
separated by multiple of eclipse year. 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 Moons 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. 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. Prediction of
solar eclipses was important to religious and political leaders.
Thales is said to have predicted the eclipse of 585BC. Predicting
solar eclipses
Slide 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.
Slide 11
Syene and Alexandria need to be aligned North-South I.e., the
Sun needs to be transiting simultaneously at these two places.
Slide 12
In 280BC, Aristarchus from the school of Alexandria measured
the distance to the Sun compared to the Earth-Moon distance. 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 Measured
angle was 87 Earth-Sun distance = 20 x Earth-Moon distance Distance
to the Sun
Slide 13
By measuring the duration of the total lunar eclipse,
Aristarchus estimated that the Moon is about 3 times smaller than
the Earth. 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). 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). Other sizes and distances
Slide 14
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 Moons Orbit: The plane of
the Moons orbit is tilted by about 5 from the plane of the Earths
orbit, or ecliptic. Key Ideas from Chapter 3
Slide 15
The line of nodes is the line where the planes of the Moons
orbit and the Earths 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 Earths shadow. During a solar
eclipse, the Earth passes through the Moons 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
Earths shadow cannot fall on the Moon and the Moons shadow cannot
fall on the Earth. Key Ideas
Slide 16
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 Earths umbra), partial (only part of the Moon passes into the
Earths umbra), or penumbral (the Moon passes only into the Earths
penumbra). Key Ideas
Slide 17
Solar Eclipses: Solar eclipses may be total, partial, or
annular. During a total solar eclipse, the Moons umbra traces out
an eclipse path over the Earths 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 Suns 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. Key Ideas
Slide 18
In summary Important Concepts Measuring the size of Earth
Measuring the size of the Moon Measuring the size of the Sun
Measuring the distance to the Sun Important Terms Corona
Coronagraph Apogee & perigee Eclipse year Saros
Chapter/sections covered in this lecture : sections 3-5 and
3-6