The Solar System

18 PART 1^ |^ EXPLORING THE SKY

Ce

les

tia

le

qu

ato

r

South

celestial

pole

North

celestial

pole

Earth

Horizon

East

West

South North

Zenith

Nadir

AURA/NOAO/NSF

The eastward rotation of Earth causes the sun, moon, and

stars to move westward in the sky as if the celestial sphere

were rotating westward around Earth. From any location on Earth

you see only half of the celestial sphere, the half above the

horizon. Thezenith marks the top of the sky above your head,

and the nadir marks the bottom of the sky directly under your feet.

The drawing at right shows the view for an observer in North

America. An observer in South America would have a dramatically

different horizon, zenith, and nadir.

The apparent pivot points are the north celestial pole and the
south celestial pole located directly above Earth’s north and
south poles. Halfway between the celestial poles lies the celestial
equator. Earth’s rotation defines the directions you use every day.
Thenorth point andsouth point are the points on the horizon
closest to the celestial poles. The east point and the west point
lie halfway between the north and south points. The celestial
equator always touches the horizon at the east and west points.

The eastward rotation of Earth causes the sun, moon, and
stars to move westward in the sky as if the celestial sphere
were rotating westward around Earth. From any location on Earth
you see only half of the celestial sphere, the half above the
horizon. Thezenithmarks the top of the sky above your head,
and the nadirmarks the bottom of the sky directly under your feet.
The drawing at right shows the view for an observer in North
America. An observer in South America would have a dramatically
different horizon, zenith, and nadir.

This time exposure of about 30 minutes shows stars as streaks,
called star trails, rising behind an observatory dome. The camera
was facing northeast to take this photo. The motion you see in the sky
depends on which direction you look, as shown at right. Looking north, you
see the Favorite Star Polaris, the North Star, located near the north
celestial pole. As the sky appears to rotate westward, Polaris hardly moves,
but other stars circle the celestial pole. Looking south from a location in
North America, you can see stars circling the south celestial pole, which is
invisible below the southern horizon.

The apparent pivot points are the north celestial pole and the
south celestial pole located directly above Earth’s north and
south poles. Halfway between the celestial poles lies the celestial
equator.Earth’s rotation defines the directions you use every day.
Thenorth pointandsouth point are the points on the horizon
closest to the celestial poles. The east point and the west point
lie halfway between the north and south points. The celestial
equator always touches the horizon at the east and west points.

1

Ursa

Minor

Ursa

Major

North

celestial

pole

Looking north

Gemini Orion

Looking east

Canis

Major

Looking south

1a