1. E ARTH ’ S S HAPE Look closely at the diameter of the Earth. Compare the measurements through the poles and the through the equator. Which one is larger?

2. E ARTH IS AN E LLIPSOID Earth’s shape is not a perfect circle because the spinning motion of the Earth’s rotation allows its shape to deform slightly. This is known as an ellipsoid ( a three- dimensional ellipse).

3. E QUATORIAL B ULGE British Scientists, led by Sir Isaac Newton, theorized that centrifugal force from Earth’s rotation caused the Earth’s equator to “spread out” from East to West. This is called an equatorial bulge.

4. T HE S EASON The Earth rotates in a counter- clockwise direction. The tilt of Earth’s rotation is 23.5 o and it is the primary cause of the seasons. This axial tilt regulates how much solar radiation the Earth receives and that cause our seasons

5. T HE ECLIPTIC P LANE The ecliptic plane outlines Earth’s orbit around the Sun. This orbit takes into account Earth’s axial tilt.

6. P RECESSION The circular wobble of Earth’s axis is called Precession. Observations indicate that the Earth’s axis of rotation is moving in a circle every 26,000 years (this is the period of rotation).

7. P RECESSION https://www.youtube.com/watch?v=Dw4Xhw4q4ec

8. W HAT CAUSES P RECESSION ? The cause of Earth’s precession is its equatorial bulge. Remember the equatorial bulge is caused by the centrifugal force of Earth’s rotation.

9. T HE W OBBLE The precession acts like a spinning top that wobbles as it rotates.

10. T HE E ARTH ’ S N UTATION There are some irregularities in Earth’s precession. The Moon and Sun pull on our planet so that in addition to precession, the Earth experiences nutation.

11. R OCKING AND S WAYING In 1748 British astronomer, James Bradley discovered nutation. Nutation is a rocking, swaying or nodding motion in Earth’s axis of rotation. This slight rocking or swaying is caused by the changing positions of the Moon. P=precession N=nutation R=rotation

12. N UTATION IS COMPLEX The motion from nutation is fairly complex. The main component comes from the Moon’s gravity. The Moon “nods or rocks” the Earth by approximately 200 meters over a 19 year cycle.

13. N UTATION AND P RECESSION A 19 YEAR CYCLE

14. T HE M OON IS TILTED TOO The Moon’s orbit drifts around the Earth and is tilted by about 5 o from the ecliptic plane. Recall that the ecliptic plane is the path of Earth’s orbit around the Sun.

15. I N S UMMARY https://www.youtube.com/watch?v=9n04SEzuvXo