Astronomy Notes
Sky mapping and motions
Name ____________________
Introduction: In order to locate objects in the night sky, one must first understand the various motions of the sky (relative to a ground observer’s frame of reference), the major reference points by which the sky is mapped and measured, and finally, the coordinate system used to map celestial objects. In this planetarium session you will experience and record data on these motions and reference points in three dimensions, which is similar to being outdoors under the real sky.
Circle the correct terms in each statement below, or fill in the missing information.
Part 1. Sky reference points
Measurements of the position of celestial objects are commonly made by recording their
altitude. This is a measure of how high above the horizon the star or planet is located. A star on the horizon has an altitude of 0 o while a star directly overhead (this point is called the zenith) has an altitude of 90 o .
1. What is the altitude of a star that is exactly halfway between the horizon and the zenith?
____
2. A preferred time to observe stars or deep sky objects is when they cross the meridian of the sky- this is when they are at their highest point and their light passes through the least amount of atmosphere. The meridian is a fixed reference line based on the observer’s location. It runs from due ___ to due ___ across the sky. When a star crosses the meridian, it is said to transit.
3. An important reference point for observers in the northern hemisphere is the location of
Polaris (the North Star). Polaris is located less than a degree off the north celestial pole. This is the place in the sky directly above the Earth’s north pole. Since the Earth rotates on its axis, and that axis may be visualized as extending from the north and south poles, it appears that Polaris remains stationary in the sky, while all other stars rotate around it.
A handy fact about Polaris is because of its unique location, the altitude of Polaris is always equal to an observer’s latitude on Earth. Estimate from the meridian: What is your latitude here in Grosse Pointe? _____
4. Think about this and answer the next 2 questions:
Where would Polaris be located in the sky if you were standing on the North Pole?
Suppose you were hijacked on a plane that flew around for many hours in an unknown direction. Upon landing, you get out of the plane, look up at the starry night sky, and realize you cannot see Polaris anywhere! Where are you?

Part 2. Sky motions
The two major motions of stars are due to the two major motions of Earth- its spin on its axis, and its orbit around the Sun. When we say a star is rising or setting, it is due to our frame of reference, assuming we are at a stationary point making the observation, when in effect we are in a moving frame of reference. Let’s examine each.
1. The daily motion of the stars rising and setting is due to Earth’s (rotation, revolution).
2. This diurnal motion causes the constellations to slowly change in the evening sky, so that the time of night determines what is visible. The true spin of Earth on its axis is not really 24 hours, but only 23 hours and 56 minutes. This causes a shift in the rising times of the stars of 4 minutes earlier each night.
So, if a star rises tonight at exactly 10 pm, it will rise next week on Thursday at ______ pm.
3. If a star transits at 11:15 pm tonight, in one month (assume 30 days), at what time will it transit? Be precise to the nearest minute, and show your work below. _____________
4. The same stars do not appear in the sky every night of the year. There is a gradual change in which constellations are visible during different times of the year. This annual motion is caused by the Earth’s orbit around the Sun, also known as its (rotation, revolution).
5. Even with both motions, it is not possible to see all the stars around the Earth from our location, because some of them are located over the south celestial pole. Likewise, because the north celestial pole is always visible in our sky, there are certain stars that may be viewed from here every clear night of the year. Stars are grouped and organized in the sky into 88 officially recognized constellations. Constellations that are visible year round from a certain location are termed circumpolar constellations. Constellations that are visible for only part of the year are known as seasonal constellations. An easy way to determine the type of constellation for a given location is to observe it through a 24 simulated period to see if it ever dips below the horizon. Observe the sky for a 24-hour period to determine if each of these constellations is circumpolar (C) or seasonal (S):
Cassiopeia ___ Lyra ___ Scorpius ___
You will now observe star motions relative to the horizon based on our latitude here in Grosse
Pointe. As the planetarium sky rotates, draw the motion of the stars as they move in each of the boxes on the next page. The first one (looking East) is done for you, as an example.
6. Generally, the stars seem to mimic the motion of the Moon and Sun through the sky, moving from east to west during the night. Where in the sky is the one exception to this?

Direction
N
Star motions from mid-north latitudes
Star Trail Description
S
E
W

Did you remember to go back and answer question 6?
Now you will travel to different places on Earth to observe how the stars appear to travel through the sky. Fill in the charts as before, then come back to answer questions.
7. From the North Pole: The North Pole has (more, less or the same amount of) circumpolar stars as Grosse Pointe.
8. True or false: The stars move in the same directions through the sky at the north and south poles.
9. True or false: The Earth rotates in opposite directions on different sides of the Equator.
10. A star that rises in the east at mid-northern latitudes will be generally in which direction 6 hours later?
11. A star that rises in the east at mid-southern latitudes will be generally in which direction 6 hours later?
12. Does the south pole also have circumpolar constellations? _____
Star motions from the North Pole
S
(any direction)
N
(any direction)
Star motions from the South Pole

Direction
N
S
E
Star motions from the Equator
Star Trail Description
W

Direction
N
Star motions from mid- south latitudes
Star Trail Description
S
E
W