|Line 258:||Line 258:|
17. At this time of year, when the Sun is under
17. At this time of year, when the Sun is under
our feet in
our feet in , where does the Gegenshein appear? Look at the frames
beginning at 013:25.
beginning at 013:25.
Under Namibian Skies is a film by Lorenzo Comolli, based on images take by him, Luigi Fontana, Giosue Ghioldi, and Emmanuele Sordini, with music by Vladimir Sterzer. It is Copyright 2011 and the version we offer in our immersive video room for this class is in high definition (HD).
As you will see when you view it, the film is based on time lapse images of the sky, and it compresses them into an experience of an entire night in a brief 15 minutes. To do this, they have selected image sequences looking in different directions over a few nights in late August and early September, 2011. It starts with a view to the west, then follows with ones to the north, then the east and finally the south. Begin this lab activity by watching it it all the way through now, full screen and in the darkened visualization room if you are in the on-campus lab. The film is also available on YouTube:
After that, we will have you consider some of the ideas it raises while you review parts of it frame by frame.
Return here after you have seen the film in its entirety.
When you viewed the film there was a date and time indicated on the bottom left. These are in "Universal Time" or "UT", the time on the prime meridian that is the reference line for geographic longitude on Earth.
1. Where is Namibia? Identify the continent it is in, above or below the equator, its latitude in degrees, east or west of the prime meridian, and longitude in hours.
2. When it is 10 PM in Namibia, what time is it where you presently are on Earth?
About 20 seconds into the film, the Sun sets in the west (notice the palm trees), and soon after the cresent Moon appears. Notice that the track of the Moon (and a hint of of the earlier track of the Sun) suggests it is coming from above you and behind you, that is, on an arc that goes back toward the north.
Shortly after this, about 40 seconds into the film, you see sunset again. Oddly now the the Sun is setting more north of those palm trees. The film identifies the date as August 24, 2011.
3. Based on the images in the film, make a reasoned estimate of what season it was when the first sunset (at 20 seconds) was filmed. Explain your reasoning!
Later that night the camera has panned more toward the southwest, and at 1 minute into the film you see Alpha Centauri, the Sun's close celestial neighbor, with the Milky way as a backdrop. Just below Alpha Centauri is Beta Centauri, the second brightest star in Centaurus. The Milky Way, our own galaxy, streaks upward from the horizon. Notice the bright object about half way toward the horizon and a little toward the south (right) at this moment. It is not a star, but its name is Omega Centauri. You may need Google to find this answer!
4. What is Omega Centauri, where is it with respect to our galaxy the Milky Way, and how much farther is it from us than the closest star, Alpha Centauri? While you answer these questions, ponder this view, with both Alpha and Omega apparently separated by only 16 degrees in this beautiful dark sky.
Moments later in the film it is the following night (now August 25, 2011) and soon after sunset. The annotation in the film points out the zodiacal light. This is dust in the plane of our solar system, scattering sunlight that we see as a faintly glowing column. To experience this almost as you would if you were there, freeze the movie at this point. Sit close to the screen so that the wide high definition (HD) field seems to surround you. The movie shows a field that is about 180 degrees across and 100 degrees high.
The "ZODIACAL LIGHT" label coincidentally is also pointing toward a star-like object. It's Saturn. The bright star above it and slightly south (left) is Spica, Alpha Virginis, the brightest star in the constellation Virgo. The bright star under the label is Arcturus, Alpha Bootes, the brightest star in Bootes.
5. Explain why Saturn appears to be in the Zodiacal Light.
At 1:15 into the film a label "Milky Way Bulge" appears. Pause at this point and consider the view you have now. The time is 20:15 UT, a few hours after sunset. The Milky Way would be overhead in the real sky, and even here, seen as an image on a flat screen, it is stunning as it spans upward from the southern horizon on your left (west is nearly in front of you). The bright star Antares, Alpha Scorpi, is below the bulge and left of the label.
6. What constellation is above the bulge, near the top of he frame? Hint: on a sky map it is east of Scorpius and it looks like a teapot. In this case, the teapot's spigot is down and its handle is up.
Let the movie run and the Milky Way will drop toward the horizon. At UT 00:00 it is nearly parallel to the horizon, the the bulge is just above one of the palm trees. The teapot is almost upside down, above the tall tree on the left.
Nearly fours hours later, at UT 03:45, Jupiter is visible as the Earth's rotation carries us around to bring it into view at the upper right. Those are the Pleiades, a cluster of stars about 400 light years from us , just above and right of Jupiter. The Pleiades are in Taurus, and most of the constellation is off the image. Jupiter is in Aries. The bright star above Jupiter close to the top of the frame is Menkar, Alpha Ceti.
7. What is that hazy patch of light, near the left side of the image at the top of the frame? The south celestial pole is just to the edge of this frame, to the left close to the top. Use Google to find its distance in light years. If Jupiter was about 30 light minutes from us at that time, how much farther is it to this hazy patch?
As the film continues it takes you to sunset of the following night, now with a view centered toward the north, with east to your left. The lights of the nearby town on the horizon at your right make a convenient reference point. At 01:56 the film labels the three bright stars of the summer triangle, a configuration of stars that is familiar to sky watchers in the northern hemisphere.
8. What are the three stars of the Summer Triangle, and what constellations are they in?
Just afterward we see a geosynchronous or "geostationary" satellite appear as it glints sunlight. Satellites like this one at a distance from Earth such that they complete an orbit around it in the same time, on average, that it takes Earth to rotate once with respect to the stars. That is, they hover over the same spot on Earth.
9. In addition to its sudden appearance and then disappearance, what is it about the geostationary satellite that tells us it is not a star? Explain.
Later in the film, at about 07:15, the camera is looking toward the roll-off-roof telescope shelter, and the film highlights the line over which geostationary satellites may appear.
10. How many geostationary satellites did you pick out? Hint: It's more than one!
At 02:24 into the film, Jupiter is seen above the northern horizon and Orion has risen int he east. It's upside down from how we would see it in the northern hemisphere (or more correctly, we are upside down compared to the familiar view when we are in Namibia). Locate Orion.
11. What is that bright reddish star on the lower right corner of the upside down constellation?
On your left, which would be toward the west because in this view you're looking north with east on your right, the Great Square of Pegasus is bove the horizon, almost directly over the second red light from the edge. Look carefully and you may be able to pick out the Andromeda Galaxy to the right of that light and 20 degrees above the horizon. In the northern hemisphere this galaxy is just visible to the unaided eye when it is overhead in the fall.
At 02:33 the film shows us the sky seen looking southeast. The south celestial pole is highlighted at 18:14 UT, just after sunset, and by 22:35 UT we see the Large and Small Magellanic Clouds (LMC and SMC) rising.
As the night goes by and morning twilight arrives around 04 UT, both of theses small neighboring galaxies are above the south celestial pole. In the frames that follow sunrise that morning, the movie shows a "star trail" view that illustrates the track of stars around the south celestial pole. Draw an imaginary line from the pole through the LMC and toward the upper left.
12. What familiar constellation is on that line, close to the upper left corner of the image? It might be easiest to pick out in the frame at 03:09, before the star trail sequence.
In the frames that follow you see the sky setting in the west, and then the view pans around toward the south again to see the Magellanic Clouds high over the pole before sunrise. The bright stars Achernar and Canopus are highlighted at 04:09 into the film, seen at 02:24 UT in Namibia that night. These are two fo the brightest southern sky stars, and Canopus is Alpha Carinae, the brightest star in Carina while Achernar is the brightest star in Eridanus. Canopus has a declination of about -52.6 degrees, that is, it is about 52.6 degrees below the celestial equator.
13. Identify a city in the United States from which Canopus would be visible. The best answer would be one where Canopus could probably be sighted easily at the right time of year.
Remember that on the first night of the film we had a faint crescent Moon just visible above the horizon at sunset. As the film progresses it spans several nights, and the camera lens has changed to show less of the sky, but more detail.
14. On 2011-08-31, what time did the Moon set (UT)? Use the palm trees as a reference to get a sense of direction. Was the moonset south of, north of , or exactly the same as the sunset direction?
Skip forward and watch the view from inside the observatory in the section that follows. By 2011-09-02 the gibbous Moon sets just before sunrise.
15. What direction is it to moonset now? Is it the same as it was when it was a crescent? Explain.
As the film continues to its conclusion, you will see again things you have noticed already, but now they will be more familiar. There are new things to look at too. Around 06:33, at 2011-08-25 20:38 UT, the two bright stars Alpha and Beta Centauri come into our view looking southwest. Moments later, the film briefly hightlights the Southern Cross. See if you can find this moment.
16. What do you see in the Milky Way, at the edge of the Southern Cross?
Beginning at about 11 minutes into the film, the camera's fisheye view shows the entire sky. At first you see the zodiacal light coming up from the western horizon, dimming as it gets higher and farther from the Sun. Then, starting at about 11:10 into the film ad at 2011-08-30 18:06 UT you'll see that this light continues across the sky, through the Milky Way and is visible quite distinctly. In this geometry, the light is called "Gegenshein", meaning "counter" shine, opposite in direction to the Sun.
17. At this time of year, when the Sun is under our feet in Nambia, where does the Gegenshein appear? Look at the frames beginning at 013:25.