Lecture 2

Question 1

Do we see stars as circumpolar at the equator?

Answer 1

No stars appear circumpolar at the equator :(

Question 2

Diurnal motion:

Answer 2

Daily rotation of the Earth

Question 3

Effects of Earth’s annual rotation around the sun: (2)

Answer 3

• Each day, stars (that rise) will rise ~4 minutes earlier than the previous day
• Each month you see different sets of constellations

Question 4

Solar day:

Answer 4

From one day to another, time it takes sun to reach meridian.

Question 5

Sidereal day:

Answer 5

Time for a specific star to return to its exact location in sky on a subsequent day

Question 6

How much shorter is the sidereal day than the solar day?

Answer 6

~4 minutes

Question 7

Cause of the difference between the solar and sidereal day:

Answer 7

Earth moves a finite distance along its orbit around the sun
each day. => Earth must rotate a little more than 360 deg to complete
one solar day

Question 8

Earth’s rotation axis is inclined:

Answer 8

23.5 degrees

Question 9

Ecliptic

Answer 9

Yearly apparent path of the sun on the celestial sphere

Question 10

Axial tilt causes:

Answer 10

Axial Tilt causes sun to appear high in the sky in our summers
(days longer), low in sky in our winters (days shorter)

Question 11

Equinox:

Answer 11

when sun’s path (ecliptic) crosses the equator

Question 12

Precession:

Answer 12

Sun & Moon exert gravitational forces on the Earth which causes the
Earth’s rotation axis to precess: the slow movement of the axis of a spinning body around another axis due to a torque (such as gravitational influence) acting to change the direction of the first axis. It is seen in the circle slowly traced out by the pole of a spinning gyroscope.

Question 13

How long does it take Earth to complete 1 cycle of precession?

Answer 13

~26,000 years

Question 14

3 effects of precession:

Answer 14

1. North star will change with time
2. Stars that are circumpolar today may not be in thousands of years
3. Some stars not currently visible in Norman (too far south) will be visible in thousands of years

Question 15

How do planets orbit the eliptic plane:

Answer 15

In a counter-clockwise motion (W->E)

Question 16

Inferior planets:

Answer 16

Mercury and Venus

Question 17

Superior planets:

Answer 17

Everything past Earth

Question 18

Apparent retrograde motion:

Answer 18

when a planet reverse its
direction in the sky (goes westwards).