Constellations are like state or country boundaries on the globe. There is nothing fundamental about them and different authorities may recognize different boundaries. They are useful because they help us find our way around the night sky. There are 88 constellations which cover the entire sky. About 80 of them can be seen from Hawaii, but not all will be visible during the spring semester. We will study 15 or so of these constellations; most have brighter stars and are fairly easy to recognize.

Background Reading: Stars & Planets, p. 5 to 10 (Constellations, Star names, and Star brightness). Additional readings for individual constellations are listed below.

For thousands of years, people looking at the night sky have grouped stars into constellations. The stars making up a constellation often seem to trace recognizable patterns. For example, the stars in Orion outline a man wearing a sword, and the stars in Maui's Fish-hook follow the shape of a fishing hook. The stars making up a constellation usually have very little to do with each other; some may be relatively close, while others are much further away. But because stars move so slowly through space, the patterns we see today have hardly changed since the dawn of history. Some of the constellations we know today were first defined by our ancestors thousands of years ago.

The pattern of stars in the sky is basically random, much like the pattern made by spattering droplets of ink on a sheet of paper. If you look at a random pattern of dots for a while, your mind will start to group dots together, and some groups might even seem like pictures of things you know. Another person looking at the same pattern might come up with some of the same groups.

It may surprise you to learn that professional astronomers don't use constellations to locate objects in the sky; instead, they use celestial coordinates. To point a modern observatory telescope at a particular object, you just give the object's celestial coordinates to a computer, and the machines do the rest. In this class we will use simple telescopes with manual controls, and a knowledge of the constellations will be helpful in finding things to observe.


It is fairly easy to recognize constellations using the sky charts in The Sky Tonight. These charts show how the sky actually appears from Hawaii at various times. Despite their circular shapes, these are not all-sky charts; each shows a wide-angle view looking north, east, south, or west. For example, chart 9-North shows the view looking north at about 20:30 (8:30 pm) in late September. The translucent overlay for each chart shows the outlines of the constellations and names some bright stars. The all-sky charts in Stars & Planets can also be used to locate constellations. You will probably want to choose the chart named for the previous month (e.g. the December chart for January observing), since we typically observe a couple of hours earlier in the evening than the 10PM used in the charts. Also remember that all-sky charts are stretched at the edge, and so distort star spacing there.

Once you've found a constellation, turn to the individual constellation charts in Stars & Planets for more detail. These charts show every star you are likely to see with your naked eye. They label stars with Greek letters (and some with numbers). These Bayer letters (see Stars & Planets, p. 8) provide standard names for stars; for example, the bright star Vega is also called alpha Lyrae. You will need to be familiar with Bayer letters to locate the stars we discuss in this class. The charts in Stars & Planets also show the brightness of each star, using a system described below, and employ special symbols to indicate double and variable stars, star clusters, nebulae, and galaxies.


A star's apparent magnitude is a number which indicates how bright the star appears in the sky. The stars were grouped by the ancient Greeks into six classes based on their visual brightness: the brightest stars were called "first magnitude", the next brightest group "second magnitude", and so on to "sixth magnitude" stars which are the faintest visible with the naked eye. This rough classification was put into a more precise framework in the 19th century, but we still have the same (somewhat counterintuitive) general idea: bright stars have small apparent magnitudes, and faint stars have large apparent magnitudes. The difference in the apparent magnitudes of two stars tells you the ratio of their brightnesses; a difference of 2.5 magnitudes implies a brightness ratio of 10:1. (Note: if you know about logarithms, you may recognize that the magnitude system employs a logarithmic scale.) To show what this means, suppose we have three stars, called A, B, and C:
star A has apparent magnitude 1.0 mA = 1.0
star B has apparent magnitude 3.5 mB = 3.5
star C has apparent magnitude 6.0 mC = 6.0
Here we are using the symbol m for apparent magnitudes; the letter written below the m indicates which star this value refers to. Then,
star A appears 10 times brighter than star B mB - mA = 2.5
star B appears 10 times brighter than star C mC - mB = 2.5
star A appears 100 times brighter than star C mC - mA = 5.0

To give some specific examples, the 'dog star' Sirius, the brightest star in the night sky, has apparent magnitude -1.4, the brightest star in Orion has apparent magnitude 0.2, and the brightest star in Cancer has apparent magnitude 3.5. With the naked eye, the faintest stars visible from Honolulu have apparent magnitudes of about 4.5 to 5.0, and the faintest stars visible from a really dark observing site have apparent magnitudes of 6.0 to 6.5.

Knowledge of apparent magnitudes is useful in making observations. For example, you might want to know how much of the constellation of Orion you can expect to see. The stars making up Orion's body have magnitudes between 0.2 and 2.2, the star representing his head has apparent magnitude 3.5, and the stars outlining his club and shield have apparent magnitudes of 4.0 or more. Thus Orion's body is easily visible, and his head is not too hard to see, but his club and shield will be harder to see unless you are looking from a really dark location. The constellation charts in Stars & Planets show stellar magnitudes by using dots of different sizes; in addition, magnitudes are usually included when individual stars are discussed in the text.


The angular separation of two objects is an angle measuring how far apart the objects appear from your point of view. For example, make a "shaka" with your arm outstretched, thumb up, and pinkie down; now imagine two lines extending from your eye to the tips of your thumb and pinkie, as shown in Fig. 1. These lines meet at an angle of about 20°, so the angular separation between the tip of your thumb and the tip of your pinkie is about 20° (this depends on the length of your arm and the size of your hand, but 20° is average). If two stars have an angular separation of 20°, you should be just about able to cover one with your thumb and the other with your pinkie by holding a shaka up to the sky at arm's length.
Fig. 1. A "handy" measure of angular separation. At arm's length, the angle between your outstretched thumb and pinkie is about 20°.

You can estimate smaller angles with your hand as well. For example, your fist, held at arm's length, defines an angle of about 10°. A pinkie finger, at arm's length, is about 1° wide and a thumb about 2°.

To measure angular separations more accurately, we will use a device called a cross-staff, which is basically a stick, 57.3 cm long, with a centimeter ruler mounted on one end. The length of the stick was deliberately chosen; if the ruler is 57.3 cm from your eye, 1 cm on the ruler defines an angle of 1°. (Note: if you know trigonometry, 57.3 = 1/tan(1°).) It's fairly easy to use a cross-staff; close one eye and place the end of the stick without the ruler just under the other eye. Sight along the stick towards the two stars you want to measure and adjust the markers on the ruler to line up with these stars. Finally, read off the positions of the markers on the ruler; the difference between them is the angular separation between the two stars.


It's often fairly easy to see a constellation when its pointed out in the sky, but harder to remember it so you can find it yourself. The best way to really learn constellations is to draw them; when you do this, your eyes often find geometrical patterns which will help you identify these constellations later. You should make your drawings to scale; this will give you a feeling for the sizes of constellations. (Many people confuse the Pleiades with the Little Dipper, not realizing that one is about ten times the size of the other!) Here's how to make an accurate drawing:
  1. Identify the constellation in the sky; we will help you with this.
  2. Pick two bright stars in the constellation, and measure their angular separation.
  3. Plot those two stars on a sheet of sketch paper, using a scale of 0.5 cm per degree (unless you can't fit the constellation on the paper at this scale.)
  4. In the margin of your sketch, name the two stars you measured, and give the angle between them in degrees.
  5. Fill in the other stars, using the ones you've already plotted as reference points.
  6. Use larger dots to indicate the brighter stars. Try to show at least three levels of brightness (e.g., bright, faint, and dim).
  7. Note any stars with distinctive colors, and any other interesting observations (e.g., "fuzzy" stars).
  8. Later, use a colored pencil to outline the shape of the constellation, and write the Bayer letter next to each star.
Make sure to include your name, the name of the constellation, the date, and the time on your sketch.


Because different constellations are visible at different times, we will return to the study of constellations throughout the semester.
  1. Early Winter (Jan 22 or Jan 29)
  2. Constellation Description in
    Stars & Planets
    Chart in
    The Sky Tonight
    Orion p. 194 1-East
    Perseus p. 202 1-North
    Cassiopeia p. 106 1-North
    Gemini p. 150 1-East
    Taurus p. 236 2-West

    All of these constellations are easy to see; you may even know some of them. Cassiopeia, Perseus, Taurus, and Orion are part of a chain of bright constellations along the Milky Way. Taurus and Gemini lie along the ecliptic, which is the path the Sun, Moon, and planets take across the sky. In observing Gemini and Perseus, take special note of the brightness of the stars beta Persei and zeta Geminorum by comparing them with other stars in these constellations.

  3. Late Winter (Feb 26 or Mar 04)
  4. Constellation Description in
    Stars & Planets
    Chart in
    The Sky Tonight
    Canis Major p. 98 2-South
    Cancer p. 94 2-East
    Auriga p. 86 2-North
    Leo p. 166 2-East
    Ursa Major p. 248 3-North

    Canis Major extends the chain of Milky Way constellations listed above; it contains the brightest star in the sky, Sirius. Cancer and Leo lie along the ecliptic. Auriga and Ursa Major are bright constellations.

  5. Early Spring (Apr 15 or Apr 22)
  6. Constellation Description in
    Stars & Planets
    Chart in
    The Sky Tonight
    Ursa Minor p. 252 4-North
    Bootes p. 88 4-East
    Virgo p. 256 4-East
    Centaurus p. 212 5-South
    Crux p. 132 5-South

    Ursa Minor, Booties, and Virgo are relatively faint, but each contains one fairly bright star of special significance. Virgo follows Leo along the ecliptic. Centaurus and Crux are spectacular constellations which can be seen from Hawaii but not from most of the continental US.


Use your text  and the smaller field map of the Orion sky area  below to gain some experience reading a star map and to answer the questions below. Turn paper copies of this chart in with your lab report. Your lab report should include all relevant observing information (date, time, location, name, etc.) and your sketch work and answers to any lab questions.

  Lab Questions:

  1. Start this exercise by picking a bright star near the eastern horizon. Identify it using your book or the full-sky chart below and estimate how many degrees above the horizon it is. Note the time of observation in your log.
  2. On your chart show the direction to the zenith and the nearest point on the horizon. Draw arrows showing the direction toward the Northern-most point in the sky and the east direction.
  3. Indicate the locations of any bright objects that are not included on the chart (missing stars, planets, moon, etc.).
  4. Circle the stars that you can see in the sky that appear on this chart.
  5. On a rough integer scale from 1 (brightest star you can see) to 6 (faintest) rank the brightness of the stars you see. Note any color differences.
  6. Draw the connecting stick figure lines for the Orion constellation to the stars you can see.
  7. Using your binoculars scan the middle region of Orion and record with a sketch and description anything that looks non-stellar.
  8. Now at the end of the lab find that star you picked out at the beginning of the lab. How far above the eastern horizon is it now? How much later is it than when you first found this star?





Last modified: January 25, 2005