How To Read A Star Chart

When you’re just starting out you need a good basic star chart that shows you where to find the bright stars and main constellations at a particular time and place.  At first, star charts are a little confusion.  So here’s how to read a star chart.

Here is a basic star chart showing the sky on April 15 at 9 p.m. from 40 degrees north latitude.  (For more charts, and a tour of the night sky through all four seasons in the northern and southern hemisphere, go to Day 3 of our mini-course in basic astronomy).

A sample star chart showing the northern sky at 9 p.m. on April 15 (click to enlarge)

The chart above tries to represent a hemispherical sky on a flat surface.  The edge of the chart represents the horizon, and the center of the chart is supposed to represent the zenith (the point directly overhead) at 40 degrees north.  East and west are reversed compared to an map of the Earth, but they will point in the right directions when you raise the map over your head.

As you learn the night sky, you will want to star charts.  To read a star chart,  here’s what you have to do…

• Find a location that’s isolated from street and house lights.  Stray light will make it harder for you to see fainter stars.  Also, for the same reason, try to avoid nights with a full moon or too much haze.

• Once you go outside, give your eyes 5 or 10 minutes to become adapted to the dark.  And to see the star charts, use a red LED flashlight or a white flashlight covered with red plastic.  The red light will preserve the sensitivity of your eye for night viewing.  (More about this on Day 5).

• Pick a direction to face, say, South, and rotate the chart so South is at the bottom.  Now raise the chart overhead.  The directions on the chart will now correspond to the directions in the sky.

• Don’t try to take in the whole sky at once.  Choose a quarter of the map, preferably one with several bright stars or a large well-known constellation like Orion or Ursa Major (of which the Big Dipper is a part).   Now, look up at the quarter of the sky that corresponds to the quarter of the map.  Make a connection with what you see in the sky with what you see on the map.  Take your time… it’s a little strange and overwhelming at first.

• Learn a few more stars at a time… don’t rush.  Once you’ve identified a few bright stars and constellations, move from what you know to what you don’t know.   Once you’ve learned most of a quarter of the sky, move to another quarter.

• While the charts are set for 9 p.m. local time, they are still useful for an hour or two on either side.  The stars will appear in about the same position, except for the stars near the horizon.  After 3 hours, the stars will have turned 1/8 of the way around the sky.  And after 6 hours, they will have turned 1/4 of the way around the sky.

• If you see an out-of-place star near the ecliptic (and in one of the constellations of the zodiac), it’s almost certainly a planet.  Since the planets move around in the sky almost daily, you will need to consult an almanac or website to figure out which planet you are seeing.  Sky and Telescope is and especially good place to check.  We also review the positions of the planets each month here at One-Minute Astronomer.

That’s all there is to it.  Well, that and a whole lot of practice.   Be patient, and savor your personal discovery of each new star and constellation.

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