My first Magazine Sky & Telescope - 01.2019

FOR F O R L ATITUDES ATIT U D ES S N E A R 40° N O R T H What’s in the sky tonight? When does the Sun set, and when does twilight end? Which planets are visible? What time does the Moon rise? Welcome to the Skygazer’s Almanac 2019, a handy chart that answers these and many other questions for every night of the year. It is plotted for skywatchers near latitude 40°° north — in the United States, the Mediterranean countries, Japan, and much of China. For any date, the chart tells the times when astronomical events occur during the night. Dates on the chart run verti- cally from top to bottom. The time of night runs horizontally, from sunset at left to sunrise at right. Find the date you want on the left side of the chart, and read across toward the right to ﬁnd the times of events. Times are labeled along the chart’s top and bottom. In exploring the chart you’ll ﬁnd that its night-to-night patterns offer many insights into the rhythms of the heavens. The Events of a Single Night To learn how to use the chart, consider some of the events of one night. We’ll pick January 6, 2019. First ﬁnd “January” and “6” at the left edge. This is one of the dates for which a string of ﬁne dots crosses the chart horizontally. Each horizontal dotted line represents the night from a Sunday eve- ning to Monday morning. The individual dots are ﬁve minutes apart. Every half hour (six dots), there is a vertical dotted line to aid in reading the hours of night at the chart’s top or bottom. On the vertical lines, one dot is equal to one day. A sweep of the eye shows that the line for the night of January 6–7 crosses many SRN: SGA19W ISBN: 9781440301087 slanting event lines. Each event line tells when something happens. The dotted line for January 6–7 begins at the heavy black curve at left, which represents the time of sunset. Reading up to the top of the chart, we ﬁnd that sunset on January 6th occurs at 4:50 p.m. Local Mean Time. (All times on the chart are Local Mean Time, which can differ from your clock time. More on this later.) Moving to the right, we see that Mars transits the meridian at 5:10 p.m., mean- ing that planet is due south and highest in the sky. The sky is still bright, but this early transit time tells us Mars will be visible later this evening as it sinks into the western sky. A tiny Moon symbol appears next on the dotted line, and the legend at the bottom of the chart indi- cates the Moon is a thin crescent tonight, setting at about 5:34. Evening twilight technically ends at 6:27, the time when the Sun is 18° below the horizon. The brightest nighttime star, Sirius, rises at 6:36, and the faint planet Uranus transits 6 minutes later. At 7:51 Polaris, the North Star, reaches upper culmination. This means it stands directly above the north celestial pole (by 39′ to 40′ this year), a good time to check the alignment of an equatorial telescope. At 8:43 p.m. comes the transit of the famous Pleiades star cluster in Taurus, followed at 10:30 by the Orion Nebula, M42. Transits of such celestial landmarks help indicate when they are best placed for viewing, and where the constellations are during the night. Running vertically down the mid- night line is a scale of hours. This shows the sidereal time (the right ascension of objects on the meridian) at midnight. On January 6–7 this is 7 h 06 m . To ﬁnd the sidereal time at any other time and date on the chart, locate that point and draw a line through it parallel to the white event lines of stars. See where your line inter- sects the sidereal-time scale at midnight. (A star’s event line enters the top of the chart at the same time of night it leaves the bottom. Sometimes one of these seg- ments is left out to avoid crowding.) Near the midnight line is a white curve labeled Equation of time weaving narrowly right and left down the chart. If you regard the midnight line as noon for a moment, this curve shows when the Sun crosses the meridian and is due south. On January 6th the Sun runs slow, transiting at 12:06 p.m. This variation is important for reading a sundial. It is caused by the tilt of Earth’s axis and the ellipticity of its orbit. Pollux transits at 12:40 a.m., as does Regulus at 3:03. Then brilliant Venus rises at 3:43, followed by Jupiter at 4:55. Four minutes later a star we usually asso- ciate with a much later season, Antares, pops up in the southeast. The ﬁrst hint of dawn — start of morning twilight — comes at 5:45 a.m. And this morning elusive Mercury rises at 6:33, well before the Sun. The Sun ﬁnally peeks above the horizon at 7:22 a.m. on January 7th. Other Charted Information Many of the year’s chief astronomical events are listed in the chart’s evening and morning margins. Some are marked on the chart itself. Conjunctions (close pairings) of two planets are indicated by a symbol on the planets’ event lines. Here, conjunc- tions are considered to occur when the planets actually appear closest in the sky (at appulse), not merely when they share the same ecliptic longitude or right ascension. Opposition of a planet, the date when it is opposite the Sun in the sky and thus visible all night, occurs roughly when its transit line crosses the Equation-of-time line (not the line for midnight). Opposi-

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