This is Dudley Observatory’s Skywatch Line for Friday, April 6 through Sunday, April 8, written by Sam Salem.
On Friday, the Sun rises at 6:29am and sets at 7:28pm; Moon rises at 12:47am and sets at 10:30am. Last quarter Moon occurs on Sunday at 3:17am. The last quarter Moon aligns with the closest apogee of the year, at 404,144 km distance. Apogee is the Moon’s most distant point from Earth in its monthly orbit. This Sunday, the half-lit last quarter Moon almost exactly aligns with lunar apogee. This is the year’s closest coincidence of quarter Moon and lunar apogee, with the two events taking place less than two hours apart.
Venus gleams at magnitude –3.9 in the early evening even as it sits low in the west at dusk. Jupiter, at magnitude –2.4, rises a little before 11pm. Mars and Saturn rise more or less together around 2:45am. The two planets are less than three degrees apart in northern Sagittarius and closely matched in brightness. Mars glows at magnitude 0.2, while Saturn is only a bit fainter at magnitude 0.5. On Saturday morning, Saturn will be the closer of these two planets to the Moon, as the Moon will pass just north of Saturn.
The 5.1-magnitude open cluster, M35, in constellation Gemini is easy to find as it’s located just above the westernmost foot of the Twins, marked by 2.9-magnitude Mu (μ) and 3.3-magnitude Eta (n) Geminorum. 10×50 binoculars show a few individual cluster members in M35. A more powerful telescope has the ability to resolve many more stars. Try to spot M35’s celestial neighbor, the 8.6-magnitude cluster NGC2158. It’s a tough catch in binoculars, but a small scope has no trouble pulling in little NGC2158 in a reasonably dark sky. The two clusters are physically similar but look quite different to us because one is much nearer to Earth than the other. M35 is the closer cluster at roughly 2,800 light-years. It’s only one-fifth the distance of NGC2158.
On April 6, 1852, Edward Sabine announced that the 11 year sunspot cycle was “absolutely identical” with the geomagnetic cycle. Sabine was an Irish geophysicist, astronomer, and explorer, who made extensive pendulum measurements to determine the shape of the Earth, and established magnetic observatories to relate sunspot activity with disturbances in terrestrial magnetism.
Sunday marks another anniversary related to sunspot discovery. On April 8 1947, the largest sunspot group recorded was observed on the Sun’s southern hemisphere. Its size was estimated at 7 billion square miles, or an area of 6100 millionths of the Sun’s visible hemisphere. Sunspots are areas of somewhat cooler surface than the surrounding solar gases, and appear as dark spots on the solar surface. Astronomers measure the sizes of sunspots as millionth fractions of the Sun’s visible area. Typically, a big sunspot measures 300 to 500 millionths. The entire surface area of the Earth is only 169 millionths of the solar disk.