Cycles of the Sky

Chapter 3:

Motions of the Planets

Earth

Venus Mercury

All planets in almost circular (elliptical) orbits around the sun, in approx. the

same plane, the ecliptic plane.

(Distances and times reproduced to scale)

The Moon is orbiting Earth in almost the same

plane.

The Annual Motion of the Sun

Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations.

The Sun’s apparent path on the sky is called the Ecliptic.

Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere.

The Seasons (I)

The Earth’s equator is

inclined against the ecliptic by

23.5o.

The different incidence angle

of the sun’s rays is causing the seasons on

Earth:

The Seasons (II)

The Seasons (III)

Northern summer = southern winter

Northern winter = southern summer

The Seasons (IV)

Sun Earth in July

Earth in January

The Earth’s distance from the sun has only a very minor influence on seasonal temperature variations.

Earth’s seasons are not because of its eccentricity.

Earth’s orbit (eccentricity greatly exaggerated)

Mercury appears, at most, ~280 from the sun.

It can occasionally be seen shortly after sunset

in the west or before sunrise in the east.

Venus appears, at most, ~460 from the sun.

It can occasionally be seen for at most a few

hours after sunset in the west or before sunrise in

the east.

Apparent Motion of the Inner Planets

Summary of Planetary Motion •  The planets all orbit the sun in the same

direction in the ecliptic plane. •  Earth is titled relative to the ecliptic plane.

–  This changes the angle the Sun’s light comes at us over the course of the year, resulting in our seasons.

•  We always see Mercury and Venus as being relatively near the Sun.

The Orbit of the Moon

As the moon orbits Earth, the same side of the moon is always pointing toward

Earth (tidally locked).

We always see

the same side of the moon!

The Phases of the Moon (I)

As the Moon orbits around Earth, we

see different portions of the Moon’s

surface lit by the sun, causing the

phases of the Moon.

Phases of the Moon •  Half of the Moon

is illuminated by the Sun and half is dark.

•  As the moon orbits Earth, we see different portions of the Moon’s surface lit by the sun, causing the phases of the moon.

The Phases of the Moon (II)

New Moon → First Quarter → Full Moon

Evening Sky

The Phases of the Moon (III)

Full Moon → Third Quarter → New Moon

Morning Sky

The Orbit of the Moon (I)

•  The Moon orbits Earth in a sidereal period of

27.32 days.

27.32 days

Earth Moon

Fixed direction in space

The Orbit of the Moon (II)

•  The moon’s synodic period (to reach the

same position relative to the sun) is 29.53 days (~ 1 month).

Fixed direction in space

Earth

Moon

Earth orbits around Sun => Direction

toward Sun changes!

29.53 days

Lunar Eclipses Earth’s shadow

consists of a zone of full shadow, the

Umbra, and a zone of partial shadow, the

Penumbra.

If the Moon passes through Earth’s full

shadow (Umbra), we see a lunar eclipse.

If the entire surface of the Moon enters the

Umbra, the lunar eclipse is total.

A Total Lunar Eclipse (I)

A Total Lunar Eclipse (II)

A total lunar eclipse can last up to 1 hour and

40 min.

During a total eclipse, the moon has a

faint, red glow, reflecting sun

light scattered in the Earth’s atmosphere.

Typically,

1 or 2

lunar eclipses

per year.

Solar Eclipses (I)

The angular diameter of the moon (~ 0.5o) is almost exactly the same as that of the sun.

This is a pure chance coincidence. The moon’s linear diameter is much smaller than that of the sun.

Solar Eclipses

Due to the equal angular diameters, the Moon can cover the Sun completely when it passes in front of

the Sun, causing a total solar eclipse.

Total Solar Eclipse

The moon’s shadow sweeps across the Earth, over points from where we

can see a solar eclipse.

Eclipses seen from space

Predicting Eclipses

Approximately 1 total solar eclipse per year

Total Solar Eclipse

During a total solar eclipse, the solar chromosphere, corona, and prominences can be seen.

The Diamond Ring Effect

Almost total, annular eclipse of May 30, 1984

Earth’s and Moon’s orbits are slightly elliptical:

Sun

Earth

Moon

(Eccentricities greatly exaggerated!)

Perihelion = position closest to the sun

Aphelion = position furthest

away from the sun

Perigee = position closest to Earth

Apogee = position furthest away from

Earth

Annular Solar Eclipses The angular sizes of the

Moon and the Sun vary,

depending on their distance from Earth.

When the Earth is near perihelion, and the Moon is

near apogee, we see an annular solar eclipse.

Perigee Apogee Perihelion Aphelion

Annular Eclipses

Conditions for Eclipses

The Moon’s orbit is inclined against the ecliptic by ~ 50.

A solar eclipse can only occur if the Moon passes a node near New Moon.

A lunar eclipse can only occur if the Moon passes a node near Full Moon.

Very Important Warning: Never observe the sun directly with your bare eyes, not even during a partial solar eclipse!

Use specially designed solar

viewing shades, solar filters, or a

projection technique.

Summary of Moon’s Motion •  One side of the moon always faces Earth,

and we see different phases as it orbits. •  If the moon, Earth, and sun line up, we see

an eclipse. –  Lunar eclipse when the moon moves into

Earth’s shadow. –  Solar eclipse when the Earth is in the moon’s

shadow. •  The moon’s orbit is elliptical, so solar

eclipses can be full or annular. •  The moon’s orbit is at an angle from the

ecliptic, so there isn’t an eclipse every month.