My first Magazine Sky & Telescope - 01.2019 | Page 21

before the last trace of penumbral shading leaves the eastern An astronaut standing inside the umbra on the Moon’s (right-hand) edge of the lunar disk? surface would see the Sun entirely hidden by Earth. But Enlargement of the umbra: Observers have long real- viewed from inside the penumbra, the Sun’s disk would be ized that the umbra’s diameter averages about 2% larger than only partly covered — varying from a small scallop as the called for strictly by the geometry of the eclipse begins (with an imperceptible dim- ming of the lunar landscape) to only a sliver The umbra’s diameter av- eclipse, and yet it can vary from eclipse to eclipse for reasons that are not fully under- of sunlight just before totality, with the sur- erages about 2% larger stood. The most likely explanation is that it roundings noticeably darker. than called for strictly is due to the varying overall transparency of Hence, the outermost part of the pen- by the geometry of the Earth’s upper atmosphere. umbra is so lightly shaded that initially eclipse, and yet it can The amount of enlargement during this you won’t notice any change in the Moon’s vary from eclipse to upcoming eclipse can be deduced after the appearance at all. But by the time the Moon’s eclipse for reasons that fact from careful telescope-aided timings of leading edge gets about two-thirds across are not fully understood. when lunar craters enter or leave the umbra. the penumbra, about 40 minutes later, you If your sky is clear, the edge of the advancing might begin to detect a very weak dimming or retreating umbra is sharp enough to permit crater timings of the Moon’s western limb (the left edge for northerners) — accurate to a few tenths of a minute. the fi rst visible sign that an eclipse is in progress. If you’d like to try this, Senior Contributing Editor Roger How early can you see this with certainty? As the Moon Sinnott has prepared predictions of the entrance and exit advances deeper into the penumbra, the shading becomes times for 24 prominent lunar features (listed on page 20). much stronger and more obvious. Pay attention to the pen- Note that these are given in Universal Time, so be sure to umbra’s color (brown? gray?) and its overall darkness. make the correct offset for your time zone. Also, before the With the full Moon so high up this January as seen from eclipse begins, I would advise anyone unfamiliar with the the U.S. and Canada, an early penumbral sighting could be Moon’s features to identify these locations on a lunar map. especially problematic due to the lunar disk’s overpowering Just before the eclipse, be sure to set your watch accurately glare. It might help to wear dark glasses — really! One trick from a cell-phone time app (such as Emerald Time) that I’ve used in the past is to project the Moon’s image with bin- displays to the nearest second. The listed times refer to each oculars or a telescope onto white paper or cardboard. crater’s center, so start to watch a given feature several min- Partial stages: The partial eclipse begins when the much utes before the prediction. darker umbral shadow fi rst touches the lunar limb. The Record your timings to the nearest 5 seconds, even though change is dramatic: A dark dent forms on the Moon’s west- your uncertainty might be 10 to 20 seconds or more. Once ern side, and as the minutes tick by the dent becomes a big, the eclipse is over, you can email your timings to Sinnott at rounded bite. The curved edge of the shadow gives a naked- [email protected] to aid his ongoing analysis of eye indication that Earth is nearly spherical (no shape other these events. In your report, mention your full name, tele- than a sphere always casts a circular shadow). scope aperture, magnification, and sky conditions. During the eclipse’s partial stages, you’ll easily see the The total eclipse: The umbra appears quite dark during penumbra as a dusky border to the shadow’s dark core. After the opening stages of partial eclipse (or the late stages after the last bit of umbra disappears, how much time elapses uar a br Entire eclipse visible e Mid- M -eclipse a t z eni enitth r in of Daytime D (Moon not up) t WORLD VIEW This map shows where the Moon will rise or set during stages of the eclipse — and, since an eclipsed Moon is always full, the Sun sets or rises at almost the same time on the opposite hori- zon. For January’s event, the sky is dark and the Moon well up across both North and South America during all phases of the eclipse. sk yandtele scope.com • JA N UA RY 2 019 19