Earth-Moon-Sun System Notes

Earth-Moon-Sun System

Earth's Motions

• The two main motions of Earth are rotation and revolution.

Rotation

• Earth’s rotation is Earth’s spin around its axis.

• It takes 24 hours.

• The main results of Earth’s rotation are day and night.

Solar Day

• Solar day is the time it takes for the Earth to rotate around its axis so that the sun appears in the same position in the sky.

• It takes 24 hours.

Sidereal Day

• Sidereal day is the time it takes for the Earth to rotate around its axis so that a distant star other than the sun appears in the same position in the sky.

• It takes 23 hours 56 minutes 4 seconds.

Why the 4 minutes difference?

• Earth orbits the Sun while rotating, moving a little bit on its orbit every day.

• Earth gets in the same position relative to the celestial stars in a shorter time.

Revolution

• Earth’s revolution is Earth’s orbit around the sun.

• It takes 365 days.

• Earth’s revolution is elliptical (not a perfect circle).

Perihelion

• Earth is closest to the sun.

• Occurs about January 3 each year.

Aphelion

• Earth is farthest from the sun.

• Occurs about July 4 each year.

• Prefixes:

  • ap-, apo- ('away from') for the farthest

  • peri- ('near') for the closest point to the primary body

• Suffix:

  • Helion- means sun or solar

Temperature Variation

• Earth is at its coolest at perihelion because rocks heat up much more quickly than water.

• Most of the landmass on Earth is in the Northern Hemisphere, and most of the Southern Hemisphere is ocean.

• The difference in solar radiation is small between those months compared to the larger variation caused by the tilt of the earth's axis.

The Moon

Lunar Phases

• Lunar phases are a result of the motion of the moon and the sunlight that is reflected from its surface

• Half of the moon is illuminated at all times, but to an observer on Earth, the percentage of the bright side that is visible depends on the location of the moon with respect to the sun and Earth

Phases Order:

• New moon

• About two days after the new moon, a thin sliver (crescent phase) appears low in the western sky just after sunset.

• During the following week, the lighted portion of the moon visible from Earth increases (waxing) to a half circle (first-quarter phase) and can be seen from about noon to midnight.

• In another week, the complete disk (full-moon phase) can be seen rising in the east as the sun is sinking in the west.

• During the next two weeks, the percentage of the moon that can be seen steadily declines (waning), until the moon disappears altogether (new-moon phase)

• The cycle soon begins again with the reappearance of the crescent moon

New Moon

• When the moon lies between the sun and Earth, none of its bright side faces Earth.

Full Moon

• When the moon lies on the side of Earth opposite the sun, all of its lighted side faces Earth.

Moon phases

• Moon phases are the same everywhere on Earth, but people north and south of the equator do see the Moon’s current phase from different angles.

Sidereal vs Synodic Month

• Sidereal month: the time it takes the moon to orbit Earth and return to the same position (360 degrees). (27.3 days)

• Synodic month: the time it takes the moon to orbit Earth and return to the same phase (even if it’s not the same position). (29.5 days)

Tidal Locking

• The moon rotates while it orbits Earth.

• The amount of time it takes the moon to complete a revolution on its axis is the same it takes to circle our planet — about 27 days.

• As a result, the same lunar hemisphere always faces Earth.

• This phenomena is called tidal locking.

Moon Illusion

• The Moon is the same width near the horizon as when it's high in the sky, but that's not what we perceive with our eyes.

• It is an illusion rooted in the way our brains process visual information.

Moon's Effect on Earth

• The Moon’s gravity keeps our planet from wobbling on its axis too much, which helps to stabilize our climate.

• The Moon also plays an important role in creating tides in Earth’s oceans.

Seasons

• The earth spins around an imaginary pole called its axis.

• The angle of the tilt is $23.5$ degrees.

• If it weren’t for the earth’s tilt, we would have no seasons.

Solstices

Summer Solstice

• When the North pole is tilted toward the sun

• Northern hemisphere’s longest day

• Occurs on June 21 or 22

Winter Solstice

• When the North pole is tilted away from the sun

• Northern hemisphere’s shortest day

• Occurs on December 21 or 22

Equinoxes

Autumnal Equinox

• Occurs during the fall

• Occurs on September 21 or 22

Vernal Equinox

• Occurs during the spring

• Occurs on March 21 or 22

• The northern and southern hemisphere have opposite seasons, solstices and equinoxes.

• Whenever the hemisphere is not specified in a sentence, then that sentence is referring to the northern hemisphere.

The Ecliptic

• The ecliptic is the joint positions of the sun as seen from Earth over 1 year.

• The ecliptic and earth’s equatorial plane intersect at equinox because the sun would be parallel to the equator in that time.

Eclipses

The Moon’s Orbital Plane

• The moon’s orbit is elliptical, its distance to Earth varies by about 6%.

Perigee

• The moon is closest to Earth.

Apogee

• The moon is furthest from Earth.

Eclipses Formation

• When the moon moves in a line directly between Earth and the sun, it casts a dark shadow on Earth.

• Eclipses do not occur every new or full moon.

• The moon’s orbit is inclined about 5 degrees to the plane that contains Earth and the sun.

• When the orbital plane of the moon intersects with the ecliptic (the path of the sun), an eclipse occur.

Solar Eclipse

• When a new moon falls exactly along the plane of Earth’s orbit and the ecliptic, it produces a solar eclipse.

• During a total solar eclipse, the moon casts a circular shadow on Earth.

• Anyone observing in this region will see the moon slowly block the sun from view and the sky darken.

• When the eclipse is almost complete, the temperature sharply drops a few degrees.

• The solar disk is completely blocked for seven minutes at the most.

Umbra

• Total solar eclipses are visible only to people in the dark part of the moon’s shadow known as the umbra.

Penumbra

• A partial eclipse is seen by those in the light portion of the shadow, known as the penumbra.

Dangers of looking directly at the sun

• Looking directly at the sun can cause permanent eye damage in less than a minute.

Lunar Eclipse

• During a total lunar eclipse, Earth’s circular shadow can be seen moving slowly across the disk of the full moon.

• When totally eclipsed, the moon is completely within Earth’s shadow.

Coppery Moon

• During a lunar eclipse, the Moon turns red because the only sunlight reaching the Moon passes through Earth's atmosphere.

• The more dust or clouds in Earth's atmosphere during the eclipse, the redder the Moon will appear.

• A total eclipse of the moon can last up to four hours and is visible to anyone on the side of Earth facing the moon.

Lunar vs Solar Eclipse