1. Christian Brothers University
Astronomy Slides 7/20/2017
Dr. Johnny B. Holmes
Professor of Physics
Christian Brothers University

2. The Tilt of the Earth and the Signs of the Zodiac (view is looking down above the North Pole; this is opposite of looking up from the Northern Hemisphere)
Virgo
Leo
Cancer
Libra
Scorpius
Gemini
spring equinox
March 21
Dec. 21
June 21
Taurus
Sagittarius
Aries
Capricornus
Aquarius
Pisces

3. Rotation about the Sun
Earth rotates counterclockwise as viewed from above North pole
Sunrise and star rise
Star rise comes before sunrise

4. 1. Phases of the Moon Sizes are NOT drawn to scale! 1
View is looking down from above the North
1st quarter
sunset
Full Moon
midnight
noon
sunrise
New Moon
3rd quarter

5. Motions of the Moon Sizes are NOT drawn to scale
View is looking down from above the North Pole
Both the earth and the Moon rotate counterclockwise as viewed looking down from above the North Pole.
1st quarter
evening
Full
midnight
noon
New
morning
3rd quarter

6. Synodic versus Sidereal
Sizes are NOT drawn to scale. View is looking down from above the North Pole. Earth is really moving down (counterclockwise), but it appears that the sun moves up (clockwise).
The sidereal period for the Moon is 27.3 days but the synodic period for the Moon is 29.5 days (a moonth).
Position of sun one moonth later.
1st quarter
evening
Full
midnight
noon
New
morning
Original Position of the sun
3rd quarter

7. 2. Shadows: Umbra and Penumbra
The umbra is the area of total shadow.
The penumbra is the area of partial shadow.
If (part of) the earth goes through the umbra of the Moon, we will have a total eclipse of the sun.
If the Moon goes through the umbra of the earth, we will see either a partial or total eclipse of the moon.
penumbra
umbra

8. Shadows: Umbra and Penumbra
The umbra is the area of total shadow.
The penumbra is the area of partial shadow.
If the earth goes through the penumbra of the Moon, we will have a partial eclipse of the sun.
If the Moon goes through the penumbra of the earth, we will only see a slight dimming of the moon – nothing particularly striking.
penumbra
umbra

9. 3. Motions of the Moon
Sizes are NOT drawn to scale. View is looking down from above the North Pole. Does the Earth’s umbra reach the moon?
Earth’s umbra does reach the moon, but since it narrows down it is only 72% the size of the earth at that distance.
1st quarter
evening
Full
midnight
noon
New
morning
3rd quarter

10. Elliptical Orbit of the Moon
Sizes are NOT drawn to scale. View is looking down from above the North Pole. Does the Moon’s umbra reach the earth? The Moon’s umbra is 373,400 km.
The Moon orbits in a ellipse rather than a circle with an apogee of 406,686 km and a perigee of 356,412 km. So sometimes the Moon’s umbra does not reach all the way to the Earth! In this case there is an annular eclipse, not a total one.
1st quarter
evening
Full
midnight
noon
New
morning
3rd quarter

11. 4. Tilt of the Moon’s orbit
The Moon orbits the earth on a slightly tilted plane relative to the plane the earth moves in as it orbits the sun (the ecliptic). The Moon’s orbit plane is tilted 5.5o relative to the earth’s plane.
This allows lunar and solar eclipses for only a short time every 5.8 months.
5.5o

12. Speed of the Moon and the speed of the path of the eclipse
At the equator, the earth is spinning and moving its circumference (25,000 miles) in one day (24 hrs) so the surface of the earth at the equator is moving about 1,000 mph towards the East which means the spot of the eclipse would be moving West.
However, the Moon is also moving towards the East so the eclipse spot would move towards the East on the earth’s surface Which speed wins?
evening
midnight
noon
morning

13. Distance to the Moon
Given that earth is the blue circle, where is the moon?
a b c d off the screen

14. Speed of the Moon and the speed of the path of the eclipse
The moon is about 250,000 miles away from the earth, (about 10 times its circumference or 30 times its diameter) so it and its spot are moving to the East at a speed of about 2π*250,000 miles per 27.3 days (a sidereal month) = 2,400 mph.
The coming eclipse will last 1 hour and 33 minutes in going from Oregon to South Carolina – about 2,000 mph East!
evening
midnight
noon
New
morning

15. A Moon “Day” 1 The Sizes are NOT drawn to scale!
View is looking down from above the North
The Moon keeps the same face towards the earth at all times.
1st quarter: mountain at sunrise
Full Moon: mountain at noon
sunset
midnight
noon
New Moon: mountain at midnight
sunrise
3rd quarter: moutain at sunset

16. Surface of the Moon Earth facing side of moon:

17. Surface of the Moon Far side of moon:

18. (diagram is NOT drawn to scale)
Tides
What causes tides?
From the previous data, and from Newton’s Laws of Gravity, we can understand that gravity – both from the sun and from the Moon – cause the tides. Looking down from above the North pole, we see:
(diagram is NOT drawn to scale) LT HT HT LT

19. “Inferior” Planets
An “inferior” planet is one who’s orbit is inside the earth’s orbit. The inferior planet moves faster around its orbit than the earth since it is closer to the sun and so has to move faster to prevent it from falling into the sun.
The diagram below is from the perspective as being viewed from above the North pole. The orbit of the planet, the earth, and the earth’s spin are all counterclockwise when viewed from above the North pole.
Maximum Eastern elongation – planet is “east” of the sun in the evening
earth
evening
Inferior conjuction
Superior conjunction
midnight
noon
morning
Maximum Western elongation – planet is “west” of the sun in the morning

20. “Superior” planets Superior planet earth
Western Quadrature – planet is “west” of the sun in the morning by 90o.
midnight
evening
morning
90o
noon
earth
Superior planet
opposition
conjunction
90o
Eastern Quadrature – planet is “east” of the sun in the evening by 90o.