Sponge: What two factors cause the seasons on Earth?

The constellations change with the seasons because the night side of Earth faces a different set at different times of the year.

The time required for the constellations to complete one cycle around the sky is called a sidereal year and is 365.256 solar days long.

The twelve constellations the Sun passes through is called the zodiac.

The moon goes through a repeating set of phases.

Phases of the moon:

• New – none of the visible surface of the moon is illuminated.

• Crescent – less than half of the visible surface is illuminated.

• Quarter - half of the visible surface is illuminated.• Gibbous - less than half of the visible surface is

illuminated.• Full - all of the visible surface of the moon is

illuminated.

The moon cycle begins when the moon is new, none of the visible surface is illuminated. Then the moon waxes (grows), first being a crescent moon.

The illuminated visible surface continues to wax until half of the visible surface is illuminated. This is the first quarter.

(It is called a quarter moon because we see ¼ of the total surface of the moon illuminated.)

The moon continues to wax (grow) beyond the quarter moon so more than half of the visible surface is illuminated. This is a gibbous moon.

The gibbous moon continues to wax until the entire visible surface is illuminated. This is a full moon.

At this point, the visible illuminated surface begins to shrink. This is called waning and produces a gibbous moon. This is called a waning gibbous.

The gibbous moon continues to wane until it forms the last quarter. It then becomes a waning crescent and continues to wane until it returns to the new moon phase.

Sidereal month -(27.3 days) moon returns to the same position on the celestial sphere.

Synodic month -(29.5 days) moon returns to the same phase. The synodic month is longer because Earth moves around the Sun.

Eclipses occur when the moon blocks the sun during the day and the moon’s shadow crosses over the earth or when the shadow of the earth crosses over the moon.

A lunar eclipse occurs when the Sun and Moon on opposite sides of the Earth. The Earth’s shadow falls on the Moon.

A lunar eclipse may be partial or total. Lunar eclipses last for approximately 100 minutes.

During a lunar eclipse the moon is a reddish color because some sunlight is bent by the Earth’s atmosphere and reaches the Moon. This light is reddish for the same reason that the sunset is red and orange.

A solar eclipse occurs when the Sun and Moon are in the same direction relative to the Earth.

In a solar eclipse the moon’s shadow falls on the earth.

The umbra is a part of this shadow on the earth. It is the area of totality, where all of the light from the sun is blocked. This total eclipse lasts for 7.5 minutes.

The rest of the shadow is called the penumbra. The penumbra is the area of partial eclipse. It is the area where not all of the light form the sun is blocked by the moon.

An annular eclipse is an eclipse where the umbra never reaches Earth. The sun appears as a ring around the moon in its new moon phase.

The fact that there are annular eclipses shows that the distance of the Moon from the Earth is not constant. The moon gets farther away from and closer to the earth over the course of its orbit.

The moon’s orbit is inclined to the ecliptic, or there would be eclipses every month.

The line of nodes is where the orbits of the sun and moon cross and thus marks the favorable seasons for eclipses. These eclipse seasons occur twice each year.

The Sun’s gravitational pull on the Moon “messes” with the Moon’s orbital orientation. This makes the moon’s orbit “wobble” so the nodes, the points where the moon’s orbit and the earth’s orbit cross, move over time.

This causes the regression of nodes.

This causes the regression of nodes.

It is a remarkable coincidence that the Sun and Moon occupy the same size on the celestial sphere, 1/2 of one degree. If this were not so, we would not see these eclipses.

The Earth’s axial tilt varies between 22 and 24 ° because of the gravitational pull of the Moon and Jupiter.

This also causes the Earth’s axis to change direction over time. This is called precession.

As a result of this “wobble’ of the axis, the vernal equinox drifts slowly around the zodiac.

Despite the song, we are actually not in the “Age of Aquarius” just yet.

This will also cause the north star to change. Today it is Polaris. In 3000 B.C., it was Thubin. In 14000 A.D., it will be Vega.