Chapter 3: Cycles of the Sky
Motions of the Planets Mercury Venus Earth All planets in almost circular (elliptical) orbits around the sun, in approx. the same plane, the ecliptic plane. The Moon is orbiting Earth in almost the same plane. (Distances and times reproduced to scale)
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 23.5 o. 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) Earth in January Earth s orbit (eccentricity greatly exaggerated) Sun Earth in July 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.
Apparent Motion of the Inner Planets Mercury appears, at most, ~28 0 from the sun. It can occasionally be seen shortly after sunset in the west or before sunrise in the east. Venus appears, at most, ~46 0 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.
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.
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. 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. Moon 27.32 days 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 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 (~ 0.5 o ) 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: Perihelion = position closest to the sun Earth Apogee = position furthest away from Earth Sun Perigee = position closest to Earth Moon (Eccentricities greatly exaggerated!) Aphelion = position furthest away from the sun
Annular Solar Eclipses Perigee Apogee Perihelion Aphelion 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.
Annular Eclipses
Conditions for Eclipses The Moon s orbit is inclined against the ecliptic by ~ 5 0. 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.