Motions of the Earth and Sky Part II

Depending on the relative sizes and distances of the Sun and a moon, you might see an eclipse like this: Sun planet moon Solar Eclipses

Or you might see an eclipse like this: Sun planet moon Solar Eclipses

In our case, the Sun is 400 times larger than the Moon, and coincidentally is also 400 times farther away, so they happen to have the same size on the sky: Sun planet moon Solar Eclipses

Solar Eclipses

Total Solar Eclipse During a total solar eclipse, only the outer atmosphere (corona) of the Sun is visible.

Solar Eclipses

Partial Solar Eclipse If you’re on the edge of an eclipse path and only a slice of the Sun is blocked by the Moon, it’s a partial eclipse. These are not very exciting, since the uneclipsed part of the Sun is still extremely bright.

The Moon’s orbit is not a perfect circle, so its distance from the Earth varies by a small amount, and therefore the Moon’s size in the sky varies too.

Annular Eclipses Because the Moon’s orbit about the earth is not perfectly circular, it is sometimes too far away to block out the entire Sun. This is an annular eclipse. It is rarer than a total solar eclipse.

Lunar Eclipses A lunar eclipse occurs when the Moon falls within Earth’s shadow. They always occur during a full moon, and can be seen from anywhere on the half of the Earth facing the Moon (i.e., the night side of Earth).

Eclipses don’t occur during every new and full Moon because the Moon’s orbital plane is tilted relative to Earth’s orbital plane. Because of that tilt, the Moon is usually above or below the Sun during a new Moon (no solar eclipse) and above or below the Earth’s shadow during a full Moon (no lunar eclipse). Shadows of the Earth and Moon

The diameter of the Moon’s shadow on the Earth’s surface during a solar eclipse is 269 km. –At the equator, the shadow moves at 1730 km/hr. –A total solar eclipse can last as long as 7½ minutes. A total solar eclipse occurs about once every 18 months somewhere in the world. At any given location, a total solar eclipse occurs once every 360 years. –The next total solar eclipse in the U.S. is on Aug Lunar eclipses happen about twice each year. They are more common than solar eclipses because Earth is larger than the Moon. It’s easy for the small Moon the fall within the large shadow of Earth, while more rare for a location on Earth to fall within the small shadow of the Moon. Frequency of Eclipses

Solar Eclipse Paths through 2020

Testing whether Earth orbits the Sun: Parallax Parallax is the apparent motion of a nearby object relative to a distant object due to the changing position of the observer.

If Earth orbits the Sun, then nearby stars should appear to move relative to distant stars over the course of a year. However, to the naked eye, the stars appear to remain fixed relative to each other. Because of the absence of noticeable parallax among the stars, ancient philosophers like Aristotle concluded that Earth must be stationary. If so, the motions of the Sun, Moon and planets across the sky must mean that they orbit Earth. This is the geocentric model of the solar system.

Observed Properties of the Original Planets To ancient observers, the planets were distinctive from the stars because they moved relative to the stars. They were called “wanderers” in Greek, from which the word “planet” is derived. The planets always stay close to the ecliptic, i.e., they move through the zodiac constellations. Ancient observers noticed two distinct types of planets. Mercury and Venus were always fairly close to the Sun the the sky (i.e., always near conjunction). They were called the inferior planets. The other planets, Mars, Jupiter, and Saturn, would appear near the Sun at one time and far from the Sun at another time (either conjunction or opposition). They were the superior planets. Planets usually move west-to-east against the fixed stars. But sometimes the planets move backwards (east-to-west). This is called retrograde motion.

Retrograde Motion Path of a planet relative to the stars

Retrograde Motion

Aristotle’s Geocentric Model (350 B.C.) Because of the apparent absence of parallax, Earth must be stationary, and the center of the solar system Sun and Moon orbit Earth To explain retrograde motion, planets must move around small circles called epicycles, which in turn orbit Earth This model could explain retrograde motion, but it didn’t do a very good job of predicting the positions of the planets the sky over time.

Ptolemy’s Geocentric Model (140 A.D.) Ptolemy revised Aristotle’s model and made it more complicated to try to improve the predictions of the positions of the planets, but it still didn’t do a great job.

Ptolemy’s Geocentric Model (140 A.D.)

Copernicus’ Heliocentric Model (1530 A.D.) Since the planets are in the heavens, Copernicus assumed that they must move in perfect circles at a constant speed. But otherwise, his model differed greatly from Aristotle’s:  The heavenly bodies do not all move around the same center.  The Earth is not at the center of the solar system. Only the Moon goes around the Earth.  The Sun is at the center of the solar system. This is the heliocentric model.  The daily motion of the Sun, Moon, and stars is due to the Earth’s rotation.  The Sun’s yearly motion is due to the Earth’s orbit round the Sun.  Retrograde motion is due to the Earth’s orbit round the Sun.

Copernicus’ Heliocentric Model (1530 A.D.)

According to Copernicus, retrograde motion is produced by parallax as Earth passes by a planet. This is the correct explanation for retrograde motion. Copernicus’ Heliocentric Model (1530 A.D.)

But the model was no better at predicting the positions of the planets than Aristotle’s model. Also, people still wondered why they couldn’t see parallax among the stars if the Earth is moving. The heliocentric model naturally explains why some planets (inferior) never stray far from the Sun in the sky. They are the planets with orbits smaller than Earth’s orbit. Copernicus’ Heliocentric Model (1530 A.D.) (answer: stars are so far away that their parallax shifts are too small to detect with the naked eye.)