Observing Projects
Moonlight is so bright that it interferes with seeing the stars. For the best view of the constellations, do your observing when the Moon is below the horizon. You can find the times of moonrise and moonset in your local newspaper or on the World Wide Web. Each monthly issue of the magazines Sky & Telescope and Astronomy includes much additional observing information.
On a clear, cloud-free night, use the star charts at the end of this book to see how many constellations of the zodiac you can identify. Which ones were easy to find? Which were difficult? Are the zodiacal constellations the most prominent ones in the sky?
Examine the star charts that are published monthly in such popular astronomy magazines as Sky & Telescope and Astronomy. How do they differ from the star charts at the end of this book? On a clear, cloud-free night, use one of these star charts to locate the celestial equator and the ecliptic. Note the inclination of the Milky Way to the ecliptic and celestial equator. The Milky Way traces out the plane of our galaxy. What do your observations tell you about the orientation of Earth and its orbit relative to the galaxy’s plane?
Suppose you wake up before dawn and want to see which constellations are in the sky. Explain how the star charts at the end of this book can be quite useful, even though chart times are given only for the evening hours. Which chart most closely depicts the sky at 4:00 a.m. on the morning that this assignment is due? Set your alarm clock for 4:00 a.m. to see if you are correct.
Use Starry Night™ to observe the diurnal motion of the sky. First, set Starry Night™ to display the sky as seen from where you live, if you have not already done so. To do this, select File > Set Home Location… (Starry Night > Set Home Location on a Macintosh) and click on the List tab to find the name of your city or town. Highlight the name and note the latitude of your location as given in the list and click the Save As Home Location button. Select Options > Other Options > Local Horizon… from the menu. In the Local Horizon Options dialog box, click on the radio button labeled Flat in the Horizon style section and click OK. For viewers in the northern hemisphere, press the “N” key (or click the N button in the Gaze section of the toolbar) to set the gaze direction to the northern sky. If your location is in the southern hemisphere, press the “S” key (or click the S button in the Gaze section of the toolbar) to set the gaze direction to the south. Select Hide Daylight under the View menu to view the present sky without daylight. Select View > Constellations > Astronomical and View > Constellations > Labels to display the constellation patterns on the sky. In the toolbar, click on the Time Flow Rate control and set the time step to 1 minute. Then click the Play button to run time forward. (The rapid motions of artificial Earth-orbiting satellites can prove irritating in this view. You can remove these satellites by clicking on View > Solar System and turning off Satellites). (a) Do the stars appear to rotate clockwise or counterclockwise? Explain this observation in terms of Earth’s rotation. (b) Are any of the stars circumpolar, that is, do they stay above your horizon for the full 24 hours of a day? If some stars at your location are circumpolar, adjust time and locate a star that moves very close to the horizon during its diurnal motion. Click the Stop button and right-click (Ctrl-click on a Macintosh) on the star and then select Show Info from the contextual menu to open the Info pane. Expand the Position in Sky layer and note the star’s declination (its N-S position on the sky with reference to a coordinate system whose zero value is the projection of Earth’s equator). (c) How is this limiting declination, above which stars are circumpolar, related to your latitude, noted above?
(d) Now center your field of view on the southern horizon (if you live in the northern hemisphere) or the northern horizon (if you live in the southern hemisphere) and click Play to resume time flow. Describe what you see. Are any of these stars circumpolar?
Use the Starry Night™ program to observe the Sun’s motion on the celestial sphere. Select Favourites > Explorations > Sun from the menu. The view shows the entire celestial sphere as if you were at the center of a transparent Earth on January 1, 2010. The view is centered upon the Sun and shows the ecliptic, the celestial equator, and the boundary and name of the constellation in which the Sun is located. With the Time Flow Rate set to 8 hours, click the Play button. Observe the Sun for a full year of simulated time. The motion of Earth in its orbit causes this apparent motion. (a) How does the Sun appear to move against the background stars? (b) What path does the Sun follow and does it ever change direction? (c) Through which constellations does the Sun appear to move over the course of a full year? In the toolbar, click the Now button to go to the current date and time. (d) In which constellation is the Sun located today? The Sun (and therefore this constellation) is high in the sky at midday. (e) Approximately how long do you think it will take for this constellation to be high in the sky at midnight?
Use Starry Night™ to demonstrate the reason for seasonal variations on Earth at mid-latitudes. A common misconception is that summertime is warmer because Earth is closer to the Sun in the summer. The real reason is that the tilt of the spin axis of Earth to its orbital plane places the Sun at a higher angle in the sky in the summer than in the winter. Thus, sunlight hits Earth’s surface at a less oblique angle in summertime than in winter, thereby depositing greater heat. Open Favourites > Explorations > Seasonal Variations to view the southern sky in daylight from Calgary, Canada, at a latitude of 51°N, on December 21, 2013, at 12:38 p.m., local standard time, when the Sun is at its highest angle on that day. Form a table of values of Sun altitude and Sun–Earth distance as a function of date in the year. To find these values, move the cursor over the Sun to reveal the Info panel. (Ensure that the relevant information is displayed by opening File > Preferences > Cursor Tracking (HUD) and clicking on Altitude and Distance from Observer.) Note the date, Sun altitude, and distance in your table. Advance the Date to March 21, 2014, and then to June 21, 2014, noting the values of these parameters. (a) On which of the three dates, in winter, spring, and summer respectively, is the Sun at the highest altitude in the Calgary sky? (b) From the values in your table, what is the Sun altitude at midday in December in Calgary? (c) How does the Sun altitude at midday on March 21 relate to the latitude of Calgary?
(d) On which of these three observing dates is Earth closest to the Sun? (e) On which of these three dates is Earth farthest from the Sun?