1
The Moon
The Phases of the Moon
• New Moon
• Quarter Moon - half of the lunar
disk is visible
• Full Moon
• Third Quarter
The Phases of the Moon
• The Moon doesn’t emit an light it shines by reflecting sunlight
• The amount of the Moon’s sunlit
face we see is dependant on the
positions of the Moon, Sun, and
Earth
2
Parallax
Measuring Distances
To determine the distances to nearby stars astronomers use the geometrical
concept of parallax.
Parallax
p
1 A.U.
=
2π × distance
360◦
p is the parallax angle measured in degrees
Parallax
distance = 1 A.U. ×
(360◦ /2π)
p
Parallax
For nearby stars we can simplify our formula for parallax by using a parsec
as a unit of distance and arcseconds as the unit of parallax angle. Then,
distance (in parsecs) =
2
1
p (in arcseconds)
Homework - Due Next Week
• Problems from the book 1.6 & 1.10
• Additional Problem: Given the following formula for parallax
distance = 1 A.U. ×
(360◦ /2π)
p
a) Calculate what 1 arcsecond is in degrees b) Determine the distance of
an object with a parallax angle of 1 arcsecond in A.U. c) What is this
distance in kilometers (1 A.U. = 1.5 × 108 km)?
Due Date Sec 6: 2/8 Sec 2: 2/9
3