My first Magazine Sky & Telescope - 04.2019

be. He noted the star’s position, and when he checked back less than three hours later, it had changed in relation to the background of “ﬁ xed stars.” He observed the “moving star” again the next night, then ﬁ red off an observing report to colleagues. In a letter that eventually made its way to William Herschel in England, Olbers indicated that he’d had dif- ﬁ culty estimating the object’s bright- ness. “The new planet appears as a star between the 5th and 6th magnitude,” he wrote, “and in the telescope, at least with the magniﬁ cations I can apply, is indistinguishable from a ﬁ xed star.” The astronomer-mathematician Carl Friedrich Gauss calculated the elements of the object’s orbit and discovered No Lyrids for Now THE LYRID METEOR SHOWER is predicted to peak on the night of April 21–22. Unfortunately, that’s not long after April 19th’s full Moon, which means the view will be compromised by moon- light. Even so, it’s worth scanning the skies for meteors. The Lyrids aren’t generally known for high meteor counts, usually number- ing some 10–20 per hour, but ﬁ reballs have been reported in the past. And there have been a few times, most notably in 1922 in Poland and 1982 in North America, when the shower pro- duced hundreds of meteors per hour. For observers in the North- ern Hemisphere, the radiant, on the border between Hercules and Lyra, is low on the northeastern horizon at dusk and continues to rise throughout the night. FOLLOW THE SHOWER ONLINE: As meteor observers report their counts to the IMO, you can watch this year’s Lyrid activity curve develop hour by hour at imo.net. that it was roughly the same distance from the Sun as Ceres (a mean of 2.8 a.u.). This led Olbers to suspect that Pallas, like Ceres, was the remnant of some destroyed planet that once orbited between Mars and Jupiter. Although we now favor theories that assign asteroids to a protoplanetary disk, it’s easy to see how the discovery of the ﬁ rst asteroids could lead astronomers toward this conclusion. For observers at mid-northern latitudes, Pallas rises in daylight so becomes visible with the darkness of mid-evening and remains so all night. On April 6th, ﬁ nd it about 23° above the eastern horizon around 9 p.m. local daylight-saving time. Pallas stands highest, about 67° high in the south- ern sky, around 2 a.m. but is still about 40° high in the west as it disappears in morning twilight. Pallas moves relatively quickly across the ﬁ eld of background stars, so you should be able to detect a change in its position over the course of just a few hours. Plan to observe it more than once, either on the same night or on the next clear evening, to verify your observations (do as Olbers did!). Several dimmer stars lie near the asteroid’s path in March and April, and a detailed p Because of the asteroid’s inclined orbit, we’ve yet to send a spacecraft to survey Pal- las. The Hubble Space Telescope and other, ground-based systems have imaged the aster- oid, but our best look at it to date comes from the Spectro-Polarimetric High-Contrast Exo- planet Research instrument on the Very Large Telescope (VLT) at the European Southern Observatory in Paranal, Chile. Pallas’s rocky, irregular globe is approximately 510 km wide. sketch will make it easier to compare the asteroid’s positions as they change over time. Eta (η) Boötis is an easy star-hop from Arcturus. Between April 8th and 12th or so, Eta serves as a good ﬁ nding aid for the asteroid. • FIND YOUR CLUB: skyandtelescope.com/astronomy- clubs-organizations . Minima of Algol 29 30 18 PERSEUS 38 Algol 21 TRIANGULUM 34 p As spring deepens, Perseus sets earlier, making April the last practical month for eve- ning observation until autumn. Every 2.7 days, Algol (Beta Persei) dips from its usual mag- nitude 2.1 to 3.4 and back. Use this chart to estimate its brightness in respect to compari- son stars of magnitude 2.1 and 3.4. Mar. UT Apr. UT 2 4 0:19 21:09 2 5 13:21 10:10 7 17:58 8 7:00 10 14:47 11 3:49 13 11:36 14 0:38 16 8:26 16 21:27 19 5:15 19 18:16 22 2:04 22 15:05 24 22:54 25 11:54 27 19:43 28 8:44 30 16:32 These geocentric predictions are from the recent heliocentric elements Min. = JD 2445641.5540+ 2.867324E, where E is any integer. For a comparison-star chart and more info, see skyandtelescope.com/algol. sk yandtele scope.com • A PR I L 2 019 49

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