Seasons, Phases of the Moon and Eclipses

1. Seasons, Phases of the Moon and Eclipses • Chapter 2.2 & 2.3

2. Reasons for the Seasons • Earth’s spin is called rotation • It’s orbit around the sun is called revolution • The Sun appears to rise in the east and set in the west because the Earth rotates west to east (counter clockwise) • The Earth also orbits CCW as well • Earth takes one year to orbit the Sun at 107,000 km/h

3. The seasons are caused by the Earth’s tilt NOT by change in distance from the Sun.

4. Reasons for the Seasons • Earth’s orbital distance does vary throughout the year- but only 3% farther at its farthest point than at its nearest. • Earth’s axis remains pointed at Polaris (also called the North star) at a 23.5 degree axis tilt.

5. Reasons for the Seasons • Summer Solstice- around June 21st, when the Northern Hemisphere receives the most direct sunlight. • Winter Solstice- around December 21st, when the Northern Hemisphere receives the least direct sunlight • Spring (vernal) Equinox- around March 21st, when N. hemisphere goes from being slightly tipped away to slightly toward the Sun. • Fall (autumnal) Equinox- around September 22nd, when N. hemisphere does from being slightly tipped toward to slightly away from the Sun.

6. Reasons for the Seasons • What is precession? • How long is a cycle of Earth’s precession? • How long will it be before the Earth’s axis will point at Vega instead of Polaris?

7. The Moon The Moon orbits the Earth about once every 27.3 days It’s average distance form Earth is 380,000km (240,000 miles)

9. The Moon • Waning means “decreasing” • Waxing means “increasing” • First quarter Moon occurs when the Moon is one quarter through its cycle. • A third quarter Moon must mean.... • Gibbous- hump-backed opposite of the Crescent moon

10. The Moon • Synchronous Rotation: the Moon must rotate once on its axis in the same time that it makes a single orbit of Earth. • This is why we always see the same side of the moon.

11. Eclipses • Lunar eclipse- when earth lies directly between the Sun and the Moon, therefore Earth’s shadow falls on the Moon. • Solar eclipse- when the Moon lies directly between the Sun and the Earth, so the Moon’s shadow falls on the Earth.

12. Eclipses • Since the Moon’s orbit is slightly inclined to the ecliptic plane, the Moon spends most of its time either above or below it. • The ecliptic plane is the plane of Earth’s orbit around the Sun. • When the Moon crosses the ecliptic plane it is called a node.

13. Eclipses • Two conditions must be met for an eclipse to occur: • The phase of the Moon must be full (lunar eclipse) or new (solar eclipse) since those are the only phases when the Sun, Earth and Moon can lie in a straight line. • The new or full moon must occur during one of the periods when the nodes of the Moon’s orbit are aligned with the Sun and the Earth.

14. Eclipses • A central umbra is where sunlight is completely blocked • A penumbra surrounds the umbra where light is only partially blocked.

15. Eclipses • A total lunar eclipse occurs when the Sun, Earth and Moon are almost perfectly aligned.

16. Eclipses • A partial lunar eclipse will occur when alignment is a bit less perfect and only part of the Moon will pass through the umbra.

17. Eclipses • A penumbral Lunar eclipse occurs if the Moon passes through the Earth’s penumbra only.

19. Total Solar Eclipse • Moon is closer to Earth in its orbit • Umbra stretches up to 270km in diameter • Prenumbral shadow up to 7000km in diameter. • People will only view a total solar eclipse if they are within the umbra.

20. Partial & Annular Solar Eclipse • Partial are seen as a result of the penumbral shadow • When Moon is farther from Earth in its orbit the umbral shadow may not reach Earth • A small region of Earth directly behind the umbra will see an annular eclipse and those in the surrounding penumbral see a partial eclipse.