2.2 - 2.4 Notes

2.2 The Reason for Seasons

What Causes the seasons?

The tilt of Earth's axis causes sunlight to fall differently on Earth at different times of the Year.

Sidereal Day

How long it takes any star to make one full circuit through our sky.

Solar Day

Our 24 hour day.

(The average time it takes the sun to make one circuit through the sky.)

Precession

A gradual wobble that changes the orientation of Earth's axis in space.

(Earth's precession takes about 26,000 years)

2.3 The Moon, Our Constant Companion

Why do we see phases of the Moon?

The phase of the Moon depends on its position relative to the Sun as it orbits Earth.

Lunar Phases

Waxing

Means Increasing

Waning

Means decreasing

(Phases from full to new moon are Waning)

(The Average Earth-Moon Distance is 380,000 km)

(Each complete cycle of phases, from one new moon to the next, takes about 29 1/2 days- hence the origin of the word month.)

What causes Eclipses?

Eclipses

When the Sun, Earth, and Moon fall into a straight line.

Lunar Eclipse

Occurs when Earth lies directly between the Sun and the Moon, so that Earth's shadow falls on the Moon.

Solar Eclipse

occurs when the Moon lies directly between the Sun and Earth, so that the Moon's shadow falls on Earth.

Nodes

The two points in each orbit at which the Moon crosses the surface.

(Eclipses can occur only during periods when the nodes line up with the Sun and Earth)

(We see an eclipse only when a full or new moon occurs at one of the points where the Moon's orbit crosses the ecliptic plane. The ecliptic is the Sun's apparent annual path around the celestial sphere.)

A lunar eclipse occurs when a Full Moon occurs at or very near one of the nodes.

A solar eclipse occurs when a New Moon occurs at or very near one of the nodes.

The shadows of the Moon or Earth consists of two distinct regions:

A central umbra is where sunlight is completely blocked.

A surrounding penumbra is where sunlight is only partially blocked.

Total Lunar Eclipse

When the moon passes through Earth's umbra.

Partial Lunar Eclipse

Only part of the Full Moon passes through the umbra.

Penumbral Lunar Eclipse

The moon passes only through Earth's penumbra.

Totality

The time during which the Moon is entirely engulfed in the umbra.

Annular Eclipse

A ring of sunlight surrounding the moon.

Saros Cycle

Eclipses recur in a cycle of about 18 years 11 1/3 days.

2.4 The Ancient Mystery of the Planets

Why was planetary motion so hard to explain?

Ancient people believed in an Earth-centered Universe. Apparent retrograde was difficult to explain because what could make planets sometimes turn around and go backward if everything moves in circles around Earth?

Apparent Retrograde Motion

Reverse course.(Retrograde means backward)

Why did the ancient Greeks reject the real explanation for planetary motion?

One of the main reasons was because of their inability to detect stellar parallax. The Greeks knew that stellar parallax should occur if Earth orbits the Sun, but they could not detect it.

Stellar Parallax

(Parallax - Apparent Shifting)