Cycles of the Sky

Chapter 3:

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)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 JulyEarth in January

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

Earth’s orbit (eccentricity greatly exaggerated)

The Motion of the Planets (I)

The planets are orbiting the sun almost exactly in the plane of the ecliptic.

The moon is orbiting Earth in almost the same plane (ecliptic).

Jupiter

MarsEarth

Venus

Mercury

Saturn

Mercury appears at most ~28º from the sun.

It can occasionally be seen shortly after sunset in the

west or before sunrise in the east.

Venus appears at most ~ 46º 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

The Tidally Locked Orbit of the moon

The moon is rotating with the

same period around its axis as it is orbiting Earth

(tidally locked).

We always see the same side of the moon facing Earth.

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.

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

Earthmoon

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 EclipsesEarth’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

sunlight scattered in

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

Prominences

Chromosphere and Corona

Diamond Ring Effect

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 EclipsesThe angular sizes of the

moon and the sun vary,

depending on their distance from Earth.

When Earth is near perihelion, and the moon is

near apogee, we see an annular solar eclipse.

Perigee Apogee Perihelion Aphelion

Almost total, annular eclipse of May 30, 1984

Approximately 1 total solar eclipse per year

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

Conditions for Eclipses (I)

The moon’s orbit is inclined against the ecliptic by ~ 5º.

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.

Conditions for Eclipses (II)

Eclipses occur in a cyclic pattern.

Saros cycle: 18 years, 11 days, 8 hours

Astronomical Influences on Earth’s Climate (I)

Factors affecting Earth’s climate:

• Eccentricity of Earth’s orbit around the sun (varies over period of ~ 100,000 years)

• Precession (Period of ~ 26,000 years)

• Inclination of Earth’s axis versus orbital plane

Milankovitch Hypothesis: Changes in all three of these aspects are responsible for

long-term global climate changes (ice ages)

Astronomical Influences on Earth’s Climate (II)

Last glaciation

End of last

glaciationPolar

regions receiving less than average energy

from the sun

Polar regions

receiving more than average energy

from the sun