A fresh face appears in the evening sky this week. Look low in the west tonight (Feb. 25) below beaming Venus, and you’ll see a delicate crescent moon with its horns tilted upward. Little devil. It moves about a fist a night upward, making a fairly close pass of Venus on Thursday, Feb. 27. Conjunctions or close approaches of the brightest planet and the moon are easy to capture in a mobile phone. Photograph them in twilight when there’s enough light to include a landmark like a scenic tree or building. Clouds in the right places can also add beauty to the scene.
Lockheed SR-71 Blackbird clip
The moon orbits the Earth with a mean velocity of 2,288 miles an hour (3,683 kph). This is nearly the same speed as the world’s fastest manned plane, the Lockheed SR-71 Blackbird, which set a record of 2,193.2 mph at Beale Air Force Base in California back in 1976. You can imagine how fast it would be to see that plane fly by. In contrast, the moon appears to move much more slowly because it’s a quarter million miles away. Plodding along at one moon-diameter per hour it covers about 12° of sky (a little more than one fist) a day as it toddles eastward in its orbit about the Earth.
One complete orbit takes 27.3 days — the moon’s orbital period. But the moon also has a synodic period of 29.5 days. That’s the time between two identical phases — full moon to full moon for example. Why the difference? During the 27.3 days the moon takes to circle the Earth, the Earth is also revolving around the sun. To return to the same phase, the Moon must continue a little farther along its orbit (2.2 days) to catch up to the same position it started from relative to the sun and Earth. That’s why its synodic period, also called a lunar month, is longer than its orbital period.
In other observing news, the star Betelgeuse has finally turned around and started to brighten again. The star normally waxes and wanes in brilliance because it pulsates (physically expands and contracts). It’s also lobbing tons of dust into space that form in its cool atmosphere from elements dredged from its core. Like a passing cloud the material can temporarily dim the star.
Astronomer Ed Guinan of Villanova University reports that Betelgeuse reached minimum light on Feb. 7-13 at magnitude 1.6 but brightened to 1.5 on Feb. 22. Many skywatchers have reported it as fainter than the neighboring star Bellatrix (magnitude 1.6). A few nights ago I estimated its light at 1.7, but last night (Feb. 24) the two stars appeared equally bright to my eye.
If you were planning that supernova party you might want to reschedule. Guinan notes that the slump coincides with the bottom of the star’s ~424-day pulsation cycle. Its unusual faintness may also be due in part to fresh dust ejected along our line of sight, which would further dim the mighty supergiant. I urge you to keep watching as more surprises may lie ahead … plus it’s just fun to watch a star change before your eyes.