solar eclipse size of sun, earth, etc

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Solar Eclipse Size of Sun, Earth, etc. Lecture 6

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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 - PowerPoint PPT Presentation

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Life in the Universe

Solar EclipseSize of Sun, Earth, etc.Lecture 6

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

Lunar umbra image taken from Mir (Russian Space Station) in Aug 11, 1999.Size = 105km and moving speed was about 1900 mph.2

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 MoonTenuous 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 Total eclipse of Aug 11, 1999 taken in Turkey4SOHO movie of grazing comets and coronal mass ejection

SOHO : SOlar and Heliospheric ObservatoryTwo sun-grazing comets plunged into the Sun in 1998 causing a coronal mass ejection5SOHO : 4 planets and Pleiades

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 EclipseWhen the Moon is at its closest position from Earth (perigee), the width of total eclipse path can 270km wide.Annular Solar Eclipse

Paths for total solar eclipses, 1997-20208Figure 3-13 Eclipse Paths for Total Eclipses, 19972020 This map shows the eclipse paths for all 18 total solar eclipses occurring from 1997 through2020. In each eclipse, the Moons shadow travels along the eclipse path ina generally eastward direction across the Earths surface. (Courtesy of FredEspenak, NASA/Goddard Space Flight Center)

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 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.

Prediction of solar eclipses was important to religious and political leaders.

Thales is said to have predicted the eclipse of 585BC.

Predicting solar eclipses

Babylonian astronomers knew about saros. However, it is most likely that they estimated saros from lunar eclipses.9

Measuring the size of EarthWell 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. 10Figure 3-14 Eratostheness Method of Determining the Diameter of the EarthAround 200 B.C., Eratosthenes used observations of the Suns position atnoon on the summer solstice to show that Alexandria and Syene wereabout 7 apart on the surface of the Earth. This angle is about one-fiftiethof a circle, so the distance between Alexandria and Syene must be aboutone-fiftieth of the Earths circumference.

Syene and Alexandria need to be aligned North-South

I.e., the Sun needs to be transiting simultaneously at these two places.11Figure 3-14 Eratostheness Method of Determining the Diameter of the EarthAround 200 B.C., Eratosthenes used observations of the Suns position atnoon on the summer solstice to show that Alexandria and Syene wereabout 7 apart on the surface of the Earth. This angle is about one-fiftiethof a circle, so the distance between Alexandria and Syene must be aboutone-fiftieth of the Earths circumference.

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 distanceDistance to the Sun

In truth, 1AU is 390 times the Earth-Moon distance. A true angle should be 89.85 degrees12By 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).Other sizes and distances

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 3The 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 IdeasUmbra 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 IdeasSolar 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 IdeasIn summaryImportant ConceptsMeasuring the size of EarthMeasuring the size of the MoonMeasuring the size of the SunMeasuring the distance to the SunImportant TermsCoronaCoronagraphApogee & perigeeEclipse yearSarosChapter/sections covered in this lecture : sections 3-5 and 3-618