# Cycles in the SkyPage 2

Slide 18

## When do we get an eclipse?

Whenever the Sun is within 18.5° of a node.

The Sun travels along the ecliptic at about 1° per day

It takes about 37 days to cross through the eclipse zone centered on each node.

A New Moon occurs every 29.5 days and therefore at least one solar eclipse must occur during each of the Sun's node crossings.

Slide 19

## Saros Cycle

“Saros” : Greek meaning “Repetition”

1 Saros = 18 years, 11 1/3 days

Line of nodes drifts westward at 19 deg / year

Eclipses repeat because the moon and the nodes return to the same place wrt the sun

The 1/3 day means you must go through 3 Saros to have an eclipse at the same location on the Earth (54 years, 1 month)

Slide 20

Slide 21

## Fun Eclipse Facts

The moon’s shadow moves at 1700 km/hour (1,048 mi/hr) .

Maximum totality is ~7 ½ minutes.

Every place on Earth will see a total solar eclipse about every 400 years.

Solar Eclipses occur more frequently than lunar eclipses ( by 5:3).

There must be at least two solar eclipses every year.

There can be two solar eclipses in back to back months with a total lunar eclipse in between.

This triple eclipse can occur twice during an eclipse year (1935, 2160).

Seven eclipses is the maximum - 4 solar, 3 lunar (1982, 2485).

Slide 22

## Will we always have total solar eclipses?

D(sun) = 870,000 mi (1.4M km)

(32.7’ to 31.6’)

D(moon) = 2,160 mi (3,476 km)

(33.5’ to 29.4’)

The moon is receding from the Earth by 3.8 cm / year.

When it has drifted another 12,552 mi (20,200 km), it will always be smaller than the sun (~1/2 billion years)

Earth’s day lengthens by 0.0016s / century

Slide 23

August 16, 1868: Helium is discovered in solar corona.

May 29, 1919: General relativity is verified

Total solar eclipses provide opportunity to study composition of corona.

Accurate timings allow calculation of solar dimensions.

Studies of ancient records reveal 0.001s slowing of Earth’s rotation

ECLIPSE SCIENCE

Slide 24

Oh leave the Wise our measures to collate One thing at least is certain, LIGHT has WEIGHT One thing is certain, and the rest debate -- Light-rays, when near the Sun, DO NOT GO STRAIGHT. - Arthur S. Eddington (1920)

1919 Solar Eclipse – Proving General Relativity

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