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

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

Image courtesy of ANL and NASA

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

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)

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:

x

y

(x1,y1)For the system to work, there must be an origin (zero point)

(0,0)

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)

Terrestrial Coordinate System

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

(http://csep10.phys.utk.edu/astr161/lect/celestial.html)

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

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

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)

(http://csep10.phys.utk.edu/astr161/lect/celestial.html)

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

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