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Astronomy

Lab: Observing Constellations

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Astronomy Lab Instructions

Astronomy Lab: Basic Observing: Knowing your way around the sky

Goal: To find our way around the sky and measure distances

Materials and tools

Procedure

  1. Use your planetarium program or one of the two online planetarium programs Your Sky or Stellarium Planetarium to look at the sky for the date and time you plan to observe. Try to schedule your observations in the first three weeks of September.

    Examples here are for 9pm Pacific Daylight Savings time, September 13, 2025, with the sky as seen from Seattle (which is probably a bit north of your own position). It should work for around 9pm anywhere in the US. Observations made two weeks before until two weeks after will have similar constellation patterns and the outer planets should be close to the locations cited, but the inner planets and the moon will be in different parts of the sky.

    To help you plan, you can quickly look up the moon's phases at the MoonConnection site (you may need to set the Calendar month and year), which gives pictures. The moon was full on September 7, o the best time for stargazing this month will after September 13 when the moon rises after sunset and is waning (unless, of course, you want to view the Moon itself!).

  2. Chose two constellations that will be above the horizon during your viewing session, one on the east or west horizon, and one near the celestial north pole.

    On September 13, 2025 at 9pm, Pisces and Aries will be rising on the Eastern horizon, and Perseus and Andromeda will be high in the northeastern sky. To the southeast, Aquarius and Capricorn will be above the horizon, with Pegasus above them (i.e., higher above the horizon, toward the zenith). Altair in the constellation Aquila is a bright star that should be due south above the horizon, and Vega in the constellation Lyra will be nearly overhead. In the southwest, Sagittarius will be setting. It lies in the middle of the Milky Way, which will be rising vertically into the sky from the southwest horizon; if it is dark enough, you may see it has a streak of white haze. In the west, the constellation Bootes with the bright star Arcturus will be setting, along with Ursa Major. That should give you some idea of the constellations you could use for this part of your viewing, but you can chose others.

    Try to avoid a constellation containing the moon unless the moon is less than 2 days old! You will try to observe both constellations, but if viewing conditions are not good for one, you may have a chance to still see the other.
  3. Check Earth and Sky Tonight or Sky and Telescope's This Week site for notes on any particularly interesting events (comets, meteor showers) likely to occur during your viewing session.
  4. Do some research on your constellations. Make a list of the
    1. brightest stars (magnitude 3 and above). Include the magnitude of the star in your notes.
    2. one star each at magnitude 4, 5, and 6 to use as "measures of seeing"
    3. location (what constellations are on its borders?)
    4. important binaries
    5. important variable stars
    6. any clusters, nebulae, or galaxies that lie within the constellation
  5. Begin by recording information about your observation session (use the forms you downloaded from the American Association of Amateur Astronomers: Observing Log and Drawing Log. What is the date and time? Where are you? What are the weather conditions (including temperature if you can determine that)? What is the cloud cover? What phase is the moon in? Has it already risen?
  6. Find your compass points on the horizon and note what landmarks (large tree, roof of house next door, distant mountain peak) are closest to north, south, east, and west points.
  7. What constellation is closest to each compass point when you start your observations? What bright stars are just above the western horizon and ready to set? What stars have just risen above the eastern horizon?
  8. Find the Big Dipper, and use it to find Polaris, the pole star. Then find the circumpolar constellations (Cepheus, Cassiopeia, Perseus, Draco, Little Dipper). Record the time of each observation and describe the location of the constellations, facing north and using the pole star as the center of a "clock face" and clock-hand locations, (e.g., Cassiopeia is at 2 o'clock from the pole star, meaning that as you look at the pole star, Cassiopeia is to the right—ease—and a little higher in the sky than the pole star). These constellations do no rise and set, but they do change position through the night.
  9. If it is not already too late, find the summer triangle stars: Vega, Deneb, and Altair. What constellation is each star in?
  10. Find Jupiter or Saturn (if they are visible). What part of the sky is each in? What constellation does it seem to be part of? Is it near any particular star, other planet, or the moon?

    In mid-September 2025, Mercury is at minimal eastern elongation and to the east (left) of the Sun. Since it is so close to the sun, the angle of the sun setting means that Mercury sets nearly at the same time, so it is never out of the sun's glare. Mars is not very far east of the sun, and sets while the sky is still full of twilight, so you not be able to view it easily for several months, until it rises ahead of the sun at dawn. Venus is west of the sun and visible before dawn. Jupiter is also a morning star at this time, best observed about 4 am when it is high above the horizon but the sky is not yet bright from the rising sun. But Saturn is to the southeast in the evening sky, so it is in a good position for view after sunset and will set long after after midnight. Observing it close to midnight when the sky is dark but it is high above the south horizon will be best. If you have a small telescope or binoculars, you should be able to spot Saturn's oblate shape from its rings.

    Planning ahead: the best planetary viewing this year for Jupiter and Saturn will be around 9pm mid-December, when both these planets will be above the hsouthern horizon and the Moon is third quarter and rising after midnight. Venus and Jupiter will be in conjunction (close together) and Mercury will be at maximum eastern elongation just after sunset at the beginning of June, 2026. Mars spends most of its time on the far side of the sun, rising or setting so close to sunset or sunrise that it is lost in the glare, in addition to being fairly far away.

  11. Can you identify any of these planets?
  12. Find your constellations. Look at each of the bright stars. Can you tell the difference in magnitudes? What is the faintest star that you can see in your constellations with the naked eye?
  13. Choose two bright stars in your constellation. Using your fingers or the palm of your hand, estimate their angular separation. Do this for at least two other pairs of stars in the sky. Make a small table of the stars and their distances.
  14. Make a map of your constellations from your own observations and label the bright stars with their names. Note the alignment of the constellation with your compass points (which part of the constellation is north now? eastward?). Are there any hazy areas that you are pretty sure are not clouds? Any small clusters of closely grouped stars?
  15. If you see any meteors, record the time, the apparent magnitude (was it as bright as the moon? Venus? one of the stars in the handle of the Big Dipper?), the length of the streak, and the constellation from which the meteor appeared to come. In general, you will see 5-10 meteors an hour before midnight, slightly more after midnight, and many more during a "shower". The best meteor-watching times are during meteor showers, when the Earth passes crosses the path of a comet and picks up debris.

    There are, unfortunately, no major showers near September 20, but there are some minor ones. The September ε (epsilon) Perseids (SPE) are a weak shower peaking on 9 September but running through 3 October. Because of the direction of the Earth's motion and rotation, these meteors appear as though they are shooting out of the constellation Perseus, which will be rising on the northeastern horizon, and are (like most meteors) more numerous between midnight and dawn. The Anthelion meteors (ANT) look like they are coming from near the border between Pisces and Aquarius, which will be in the northeastern part of the sky. The ν (nu) Eridanids (NUE) have a radiant (center of origin of appearance) ν Taurus and appear from late August to the mid-November. The η (eta) Eridanids (ERI) also come from this part of the sky. The Aurigids (AUR) look like they are coming out of the constellation Auriga (near the bright star Capella). Together, these five weak showers may produce around five meteors an hour. Compare this with the peaks listed below for the major fall meteor showers:

    • The Orionids (October 21) 20-25 meteors/hour; associated with comet Halley
    • the Taurids (November 5) 5 meteors/hour; associated with comet Encke
    • the Leonids (November 17-18) varies from dozens to thousands, 25/hour predicted for 2017; associated with comet Tempel-Tuttle
    • the Geminids (December 14) 75/hour or more; associated with object 3200 Phaethon, a "rock comet", i.e., ice-covered asteroid.
  16. Note the time you finish your observations (try to spend at least an hour on them). What constellations are at each of the compass points now? Have the stars you observed above the western horizon when you started set? Are there new stars above the eastern horizon? What has happened to the stars above the north horizon?

Report

Write a report of all your observations and post it to the Moodle assignment for this lab. Be sure to include

  1. A list of the materials and optical instruments that you actually used.
  2. A brief description of your procedure, with notes on any major changes you made to the suggested procedure above. All lab reports should include enough information about your materials and methods to allow a similarly equipped fellow student to repeat your observations.
  3. Your observing and drawing logs with their raw information (scan your drawing log if necessary).
  4. A summary of your session session information, presented in a suitable format, like a table or graph:
    1. Date, time, time zone
    2. Location (city name, park; geographic coordinates -- latitude and longitude -- if you know them).
    3. Temperature and air quality (still, windy)
    4. Cloud cover ("haze in the north, clear to the south")
    5. Moon location and phase; time of rising (check newspaper)
    6. Seeing level
  5. Answers to the questions asked in the procedure.
  6. Any observations you think interesting or important.

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