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: 10o 1o 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)

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

Circumpolar Constellation Chart (SC002) Shows constellations near the North or South Celestial Poles Shows lines of RA and Dec Good for any date/time