Navigating the Night Sky

Constellation Charts and

Planispheres

Big Questions:

What are the “guideposts” we use

when navigating the night sky?

How do Earth’s rotation and

revolution affect what we see in the

sky?

What tools can we use to locate

objects in the sky?

Celestial Sphere “Guideposts”

Horizon

Zenith

Celestial North Pole

Celestial Equator

Ecliptic

Declination

Right Ascension

Your Personal Location

Your horizon (the

red line) depends

on your location

Your latitude on

Earth is also the

Declination of your

Zenith

The Horizon and Visible Stars

The southernmost stars you can see must have a declination greater than – (90o - l)

For us: - (90o – 40o) = - 50o

So we can never see the “Southern Cross”

constellation Crux, because it has a

declination of - 60o and is always below our

horizon.

Because of Earth’s Rotation

Celestial Sphere appears to rotate clockwise

(E to W)

Stars near the equator (equatorial

constellations) appear to rise in the East and

set in the West

Stars near the poles (circumpolar

constellations) appear to circle around the

Celestial Poles

Because of Earth’s Revolution

Each day, you look in a slightly different direction

in space

Stars appear to rise 4 minutes earlier each

night.

In two weeks, the stars rise about an hour earlier

Different stars are visible in the winter vs. the

summer

In 12 months, they appear to move all the way

around the sky back to the same position

Star Trails

Equatorial stars

Circumpolar stars

Measuring the Sky Astronomers measure angular separation of objects in

degrees.

1o = 60’ (arcminutes) The full Moon is about 30′ across.

1’ = 60” (arcseconds) The resolution of a good telescope

The angular separation of any point on the horizon and the

zenith is 90o.

Angles in the Sky

Your hands and

fingers are

convenient

measuring tools.

When you hold your

hand at arm’s length,

you can estimate

angles like this:

1o 10o

5o

15o 25o

Types of “Sky Maps”

A Sky Chart for a specific

date, time, and latitude

Shows the sky

visible to an

observer at a

specific latitude

for a specific date

and time. The

circle’s edge is

the horizon, your

zenith is at the

center

A Planisphere or Star Wheel Adjustable

Rotate the wheel

until the date

matches with the

time of the

observation.

Polaris is at the

center

Align to a cardinal

point (N, S, W, E)

Circumpolar Constellation Chart

(SC002)

Shows constellations near the North or South

Celestial Poles

Shows lines of

RA and Dec

Good for any

date/time

Equatorial Constellation Chart

(SC001) Shows Declinations from -60 to +60

Good for any date/time/mid-latitude

Web interactive

For practice reading an equatorial

constellation chart, visit: http://www.physics.csbsju.edu/astro/SC1/SC1.01.html