Constellation of the Month: Sextans, the Sextant
This month's constellation is another of Johannes Hevelius' creations, the Sextant—an instrument he used often to take measurements in the sky. It is also used by navigators to measure angles in the sky and thereby determine a ship's position.
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Click map to enlarge it. Looking southeast in the evening sky, we can see Saturn shining brightly with a yellowish glow. Below it is the space occupied by the constellation Sextans. The constellation is made up of very faint stars and has no real structure. The only interesting object in the whole constellation is the Spindle Galaxy, NGC 3115. It is one of two that have this name, with the other in Draco. |
Early navigators on oceangoing ships did not have GPS, Loran or inertial navigation systems to tell them where they were. Near shore, they could look for landmarks. For centuries, ships hugged the coast so they would not get lost. Sometimes, because of a storm or on a voyage of exploration, they would lose sight of the shore. The sea looked the same in every direction, so the only thing the navigator could see that offered any clue to his position was the sky.
During the day, the Sun ruled the sky, but at night the stars filled the marine sky. Every navigator could find the "North Star," Polaris, in his sleep. Then he could use his sextant to measure the altitude of the North Star, which gave him the latitude fairly accurately. Naturally, a navigator would make not just one measurement, but a series, averaging the result. Since Polaris was 3.3 degrees from the actual pole in the 1500s (thanks to precession, that difference is now less than a degree today), the navigator also had to correct for that. The navigator would actually use many stars and average the results. Star charts of the day were an important navigational aid.
So the navigator could get the latitude easily enough, but what about the other coordinate, longitude? He could easily measure the angle of a given star above the horizon, but to convert that to longitude requires knowing the correct time. Think about the spinning Earth, with you standing on the equator looking at a star 45 degrees above the eastern horizon. As the Earth rotates eastward, the star will rise higher in the sky. After an hour it would be 15 degrees higher up (360 degrees divided by 24 hours gives 15 degrees an hour). So you must know what time it is in order to use your measurement of the star's altitude.
Most early clocks were controlled by a pendulum that took precisely the same amount of time to swing back and forth. Unfortunately, that rhythm was disrupted by the motion of the ship, making the time inaccurate. The British Admiralty offered a reward to anyone who could invent a clock that would keep accurate time. Finally, in 1735, John Harrison invented the first of four marine chronometers that could accurately keep time. This allowed navigators to calculate their longitude, which along with their latitude gave the position of their ship.
As part of the proof that Harrison's last timepiece, the H-4, actually kept accurate time, Captain James Cook took a replica of it called the K-1 along on his second voyage of discovery. During three years of sailing around the world, the daily difference of K-1 never exceeded eight seconds, corresponding to a distance of two nautical miles at the equator. Cook referred to the clock as "our faithful guide through all the vicissitudes of climates."
The Planets for April 2007
The Evening Star in April is the planet Venus. It is the first star visible in our western sky, continuing its current evening apparition. If you have been watching it since the beginning of the year, you will notice that it has moved quite a bit north of where it was in January. Venus (with the Sun following it) has been climbing up the ecliptic. In addition, Venus is two degrees north of the ecliptic. Venus is magnitude 4.1, and its 14.8 second-of-arc across disc is 74 percent illuminated. Venus starts the month in Aries and rapidly moves into Taurus, running eastward between The Bull's horns for the rest of the month.
Watch the Skies April 2, 11:15 a.m. April 7, 7 a.m. April 10, 12:04 p.m. April 17, 5:36 a.m. April 20, 2 a.m. April 24, 12:36 a.m. April 25, 4 a.m. April 26, 3 a.m. April 28, 1 p.m. |
Saturn is still in western Leo this month. It creeps up to the border of Cancer, then turns around and heads farther back into Leo. Its disc is starting to shrink again, down to 18.8 seconds-of-arc across. The rings are tilted 12.8 degrees to our line of sight with the southern face showing, and stretch 42.7 seconds-of-arc across. Saturn is magnitude 0.1, and sets around 3:30 a.m.
Jupiter rises around 11 a.m. in the constellation Ophiuchus this month. Shining at magnitude 2.5, Jupiter is 43.4 seconds-of-arc across along its equator. Along the poles, however, Jupiter is only 40.5 seconds-of-arc across. This is because Jupiter spins very rapidly, 2.4 times faster than the Earth. Centrifugal force bulges out the equator seven percent more than the poles.
Mars slips from Capricornus into Aquarius on the second day of April. It remains in Aquarius for the rest of the month. The Red Planet rises at 4:30 a.m., but its disc, though larger than last month, is still a disappointing 5.0 seconds-of-arc.
Mercury is still in the morning sky in April, very low in the east just before sunrise, at magnitude 0.5. Mercury is moving rapidly eastward. It starts the month in Aquarius, moves into Pisces, clips the northwest corner of Cetus, goes back into Pisces, and finally ends the month in Aries. Mercury's disc is getting smaller and becoming increasingly full as it slips down toward the Sun, passing behind it on May 2. It starts out 70 percent illuminated at the beginning of the month, and is full by the end.
Mars will be 0.7 degrees south of Uranus on April 28. That morning, you can see Mars, the fourth magnitude star Phi Aquarii and Uranus in a straight line, with Mars at the south end and Uranus at the north, some 51 minutes of arc (almost a degree) long. Take a peek with your telescope at this grouping. If you have never seen it before, enjoy your first view of Uranus and "keep watching the sky"!
An amateur astronomer for more than 35 years, Bert Stevens is
co-director
of Desert Moon Observatory in Las Cruces.