Chapter 3: Cycles of the Sky

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.

Earth’s equator is inclined against the ecliptic by 23.5º. 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 summer = southern winter Northern winter = southern summer

The Seasons (IV) Earth’s orbit (eccentricity greatly exaggerated) Earth’s orbit (eccentricity greatly exaggerated) Earth in January Earth in July sun Earth’s distance from the sun has only a very minor influence on seasonal temperature variations.

The Motion of the Planets (I) The planets are orbiting the sun almost exactly in the plane of the ecliptic. Jupiter Venus Mars Earth Mercury The moon is orbiting Earth in almost the same plane (ecliptic). Saturn

Apparent Motion of the Inner Planets 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.

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 moon Earth Fixed direction in space

The Orbit of the moon (II) Fixed direction in space 29.53 days Earth moon Earth orbits around sun => Direction toward sun changes! The moon’s synodic period (to reach the same position relative to the sun) is 29.53 days (~ 1 month).

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 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 Chromosphere and Corona Prominences

Diamond Ring Effect

Earth’s and moon’s orbits are slightly elliptical: Apogee = position furthest away from Earth Earth Perihelion = position closest to the sun moon Perigee = position closest to Earth sun Aphelion = position furthest away from the sun (Eccentricities greatly exaggerated!)

Annular Solar Eclipses The angular sizes of the moon and the sun vary, depending on their distance from Earth. Perigee Apogee Aphelion Perihelion When Earth is near perihelion, and the moon is near apogee, we see an annular solar eclipse.

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 glaciation Polar regions receiving less than average energy from the sun Polar regions receiving more than average energy from the sun