1. The Science of Astronomy
Astronomy involves the study of celestial objects and their interactions
It is a science much like biology, chemistry, and physics
Observations and measurements are made of objects and phenomena
Theoretical models are constructed in order to explain the observations
Comparisons are made between the theoretical models and the observations
Modifications are made to the theoretical models in order to better explain the observations
There are both observational (experimental) and theoretical astronomers/astrophysicists

2. The Science of Astronomy
Sometimes, the models used are relatively simple
Gravity is well understood and is used to explain the orbits of planets as well as spacecraft trajectories
Other times, the models require some approximations and simplifications
When there are many objects interacting with one another
When there are lots of different effects happening at the same time
Sunspots
Star and planet formation
Like other sciences, astronomy has advanced with progress in technology (like the Hubble Space Telescope) and computing power

3. Image courtesy of ANL and NASA
Nuclear astrophysics

4. The Evening Sky
We’ll focus on the universe that is “close to home”, namely our Solar System
First we’ll try to understand how the sky “works”
How do astronomers specify locations on the sky?
Why do the Sun, Moon, planets, and stars appear to move the way they do in the sky?
Why are there phases of the Moon?
Since observation is such an important part of astronomy, we’ll start with a good old fashioned stargazing session on a clear evening (no special equipment necessary!)
Where celestial objects are in the sky depend on the time and the location

5. The Evening Sky
Several distinctive patterns of stars appear (constellations)
The following observations appear consistently:
Looking northward, the familiar Big Dipper is seen
The two stars that form the “ladle”, furthest from the “handle”, form a line that points toward the star Polaris (the “North Star”)
Facing Polaris means facing due North
The stars, planets, and Moon appear to move westward during the night (the motion can be detected within about a minute of observation)
Polaris’ position appears fixed
There is a circumpolar zone of stars centered near Polaris that always remain above the horizon and appear to rotate counterclockwise about Polaris
Stars not located in the circumpolar zone move on circles that carry them below the horizon (size of circumpolar zone depends on location)

6. Coordinate Systems
While observing, it is helpful to have a definitive system for locating objects
Astronomers use several ways of locating objects in the sky
Locations are typically identified by using a coordinate system
Coordinates are a set of numbers that pinpoint a location
For example, house address “200 1st Ave.” combines a pair of numbers (200,1) to locate a house
A point in a 2–D space can be located with a coordinate system having two axes: y
For the system to work, there must be an origin (zero point)
(0,0)
(x1,y1) x

7. Terrestrial Coordinate System
The most commonly used terrestrial coordinate system (for locating places on Earth) uses an origin located on the Earth’s equator
The coordinates consist of latitude (angle between equator and geographical location) and longitude (angle, east or west, around the equator to point nearest to location)
The equator is an example of a great circle: a circle that divides a sphere into 2 equal parts (northern and southern hemisphere in this case)
For historical reasons, the exact location of the origin on the equator is due south of the former location of the Royal Observatory in Greenwich, England
The longitude line passing through Greenwich is called the prime meridian (or Greenwich meridian)

8. Terrestrial Coordinate System

9. Celestial Sphere
We would like to develop a similar coordinate system for the sky, since it appears to form the inside of a sphere from our vantage point on Earth
“Celestial sphere”
It’s an incorrect description, because the distances of celestial bodies vary enormously, but it is convenient way to describe celestial locations
The celestial sphere has a visible and invisible hemisphere from our vantage point
We see only the hemisphere above our horizon
Celestial sphere (

10. Angles
To measure distances on the imaginary celestial sphere, we use angular separations
Most common unit of measure: the degree
One degree = 1/360 of a full circle
Smaller units exist for finer measurements
One minute of arc = 1/60 of a degree
One second of arc = 1/60 of a minute of arc (equal to angular diameter of ball in tip of ballpoint pen at a distance of 100 yards!)
You can use your hands to make approximate angle measurements – at arm’s length:
Your finger is about 2° across
Your fist is about 10° across the knuckles
Your outstretched hand is about 20° across from the tip of the thumb to the tip of the little finger

11. Angular Sizes of Some Celestial Objects
Constellations: few degrees to few tens of degrees
Big Dipper is about 20° (2 fists) across
Sun: about 0.5°
Moon: about 0.5°
Important implications of the near equality of Sun and Moon angular sizes
Stars: all less than 1 sec of arc
The bowl of the Big Dipper is about 30° from Polaris
Any object directly overhead is 90° above the horizon
Any object “half-way up” in the sky is 45° above the horizon

12. The Horizon System
A good coordinate system to locate objects from a single location is the horizon system
It measures locations relative to the celestial horizon, a great circle located 900 from the point on the celestial sphere directly over your head (zenith)
Be careful to distinguish between “horizon” (the irregular line marking the meeting of sky and Earth) and “celestial horizon” (which can only be seen at sea or middle of vast plain) (
Celestial horizon (bisects celestial sphere)

13. The Horizon System
The two coordinates of the horizon system are altitude and azimuth
Altitude = angular distance above the celestial horizon (corresponds to latitude in terrestrial coordinate system)
Altitude of horizon = 0°, altitude of zenith = 90°
Azimuth = angular distance measured eastward from north, around the celestial horizon, to the point directly below the chosen point on the celestial sphere (corresponds to longitude in terrestrial coordinate system)
Azimuth of East = 90°, South = 180°, West = 270°
Coordinates depend on time and location of observation

14. Apparent Rotation of Celestial Sphere
We’ve already seen that the stars in the circumpolar region never set but move on circles centered near Polaris
Beyond the circumpolar region, the motions of stars carry them below the horizon
For an observer in the Northern Hemisphere, stars in the extreme southern part of the celestial sphere are never carried above the horizon
This also means that observers in the Southern Hemisphere would never see stars in the extreme northern part of the celestial sphere, like those in the Big Dipper