Solar Eclipse
Size of Sun, Earth, etc.
Lecture 6
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
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.
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
SOHO movie of grazing comets and coronal mass ejection
SOHO : 4 planets and Pleiades
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.
Paths for total solar eclipses, 1997-2020
Predicting solar eclipses
 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.
Prediction of solar eclipses was
important to religious and political
leaders.
Thales is said to have predicted the
eclipse of 585BC.
 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.
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.
Syene and Alexandria need to
be aligned North-South…
I.e., the Sun needs to be
transiting simultaneously at
these two places.
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
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).
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.
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.
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).
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.
In summary…
Important Concepts
Important Terms
 Measuring the size of Earth
 Measuring the size of the Moon
 Measuring the size of the Sun
 Measuring the distance to the Sun
 Corona
 Coronagraph
 Apogee & perigee
 Eclipse year
 Saros
Chapter/sections covered in this lecture : sections 3-5 and 3-6