The Solar System

CHAPTER 2 | THE SKY 19

North celestial

pole

North

celestial

pole

North

celestial

pole

North

celestial

pole

N

W E W E W E W E

Latitude 90°

Latitude 60°

Latitude 30°

L

S

S

S

S

N

N

South

celestial

pole

Latitude 0°

Latitude –30°

N

N

W E W E W E W E

S

S

S

S

N

N

N

ZenithZenith

ZenithZenith

ZenithZenith

ZenithZenith

ZenithZenith

Ursa

Minor

Ursa

Major

A few circumpolar
constellations

Polaris

Perseus

Cassiopeia

Angular distance

Rotation

of sky

Rotation

of sky

Cepheus

Astronomers measure

distance across the

sky as angles.

Circumpolar constellations are those that never rise

or set. From mid-northern latitudes, as shown at left,

you see a number of familiar constellations circling Polaris

and never dipping below the horizon. As the sky rotates, the

pointer stars at the front of the Big Dipper always point

toward Polaris. Circumpolar constellations near the south

celestial pole never rise as seen from mid-northern

latitudes. From a high latitude such as Norway, you would

have more circumpolar constellations, and from Quito,

Ecuador, located on Earth’s equator, you would have no

circumpolar constellations at all.

What you see in the sky depends on your latitude as shown at right. Imagine that you

begin a journey in the ice and snow at Earth’s North Pole with the north celestial pole

directly overhead. As you walk southward, the celestial pole moves toward the horizon, and

you can see farther into the southern sky. The angular distance from the horizon to the north

celestial pole always equals your latitude (L)—the basis for celestial navigation. As you cross

Earth’s equator, the celestial equator would pass through your zenith, and the north celestial

pole would sink below your northern horizon.

2

3

3a

Astronomers might say, “The star was only 2 degrees from

the moon.” Of course, the stars are much farther away than the

moon, but when you think of the celestial sphere, you can measure

distanceson the sky as angular distances in degrees, minutes of

arc, and seconds of arc. An arc minute is 1/60th of a degree, and

an arc second is 1/60th of a minute of arc. Then the angular

diameter of an object is the angular distance from one edge to

the other. The sun and moon are each about half a degree in

diameter, and the bowl of the Big Dipper is about 10° wide. In order

to distinguish these units from the similarly named units of time,

astronomers refer to small angles in arc minutes or arc seconds.

The word arc indicates that the numbers refer to angles and not time.