University of Michigan - Department of Astronomy

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Version: outseason intro


Out-of-Season Constellations

Where has the Pleiad gone?
Where have all the missing stars found light and home?
Who bids the Stela Mira go and come
Why sits the Pole-star lone?
And why, like banded sisters, through the air
Go in bright troops the constellations fair?

--Nathaniel Parker Willis

Overview:

Introduction

There are a total of 88 constellations officially recognized by the International Astronomical Union. Each constellation has official boundary lines to divide up the sky. Patterns that aren’t recognized officially are called asterisms. Thus, the constellations define specific regions on the sphere of the sky. How does our location on the earth affect how we see the sky? How does our viewing location limit the areas of the sky that we see?

Circumpolar constellations are those that are close enough to the North Celestial Pole that they remain above the horizon all the time as the sky turns (See Figure 1).  For example, from Ann Arbor, Ursa Major is considered circumpolar even though the bear’s feet dip below the horizon, because most of the constellation, especially the brightest stars, remain above the horizon at all times. Note that the observer's latitude on Earth determines which constellations are seen to be circumpolar. Thus, if you move substantially north or south, you will see more or fewer constellations to be circumpolar.

Seasonal constellations are the remaining non-circumpolar constellations that are visible at the observer's latitude. These constellations will rise and set as the earth turns (See Figure 1). They will be best viewed in the evening during a particular season. For example, if you go out on any evening from December through March, Orion will be high in the sky. Since December through March is winter in the northern hemisphere, Orion is a considered to be a winter constellation at most northern latitudes.

Figure 1: The area of the sky visible from Ann Arbor and similar latitudes.

Coordinate System grid

Using a Planisphere

A planisphere is a flat, two-dimensional projection of the dome of the sky showing the constellations. Most planispheres have an adjustable mask with an oval aperture that shows the portion of the sky that is visible at a given date and hour. Since the visibility of the constellations depends on latitude, planispheres are usually designed for specific latitudes. A planisphere designed for 40º N latitude will work for most of the continental United States, depending on how accurate you want it to be, near the horizon.

The mask is labeled with hours of the day, and the sky chart itself is labeled with calendar dates. To set your planisphere to the desired date and time, rotate the mask so that your viewing time aligns with the date on the star chart. For example, rotate the mask until 10 PM and January 1 align to see what the sky looks like on January 1st at 10 PM.

The mask is also labeled with the cardinal directions, North, East, South, West. Note that East and West are reversed. To see why, hold the planisphere over your head with North oriented correctly. East and West will now point in the correct directions, and the stars on the chart should match up with their real positions in the sky.

The meridian is the line that passes through the zenith (overhead) and through the poles. Some planispheres also show the meridian on the mask.  If not, the meridian should run from North to South through the middle of the mask's aperture, crossing the north celestial pole.  You can glue a piece of string across the mask to mark the meridian. Objects are highest in the sky when they are on the meridian. To determine the season in which a constellation is best viewed at a given time of night, rotate the mask until the constellation is on the meridian at the desired hour, and then read the corresponding date.

Activities


Updated: 7/9/10 by SAM & MSO

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