September 2016 – Vol. 29 No. 1

Celestial Highlights for April 2013

Posted: Monday, April 1st, 2013

by Robert Victor and Robert D. Miller

We invite you and your students to use the accompanying evening twilight sky chart for April 2013 to identify Jupiter and the brighter stars as they first appear after sunset. Begin observing no later than one-half hour after sunset, or even earlier when the Moon is visible. 

Robert D. Miller is kindly providing a pair of sky charts for each month. One chart tracks positions at dusk, and the other at dawn, of the five naked-eye planets and the sixteen stars of first magnitude or brighter visible from California. Positions of the stars and planets are plotted each day at mid-twilight, when the Sun is 9° below the horizon. Locations of planets are plotted as a separate dot for each day, with bolder dots plotted weekly on the 1st, 8th, 15th, 22nd, and 29th day of the month (Mondays in April). Star positions are plotted as continuous tracks, with all stars drifting westward (left to right on the charts) in the course of the month, owing to the Earth’s revolution around the Sun.

For Los Angeles, Palm Springs, and other places near lat. 34° N, evening mid-twilight during April occurs about 40-42 minutes after sunset, and morning mid-twilight occurs at a similar interval before sunrise. For northern California, mid-twilight from lat. 40° N in April occurs about 43-47 minutes after sunset or before sunrise.

Sometimes a star or planet is below the horizon at the start of a month, but may appear above the eastern horizon during the course of the month. For example, Spica first appears in evening mid-twilight south of east around April 4, and Saturn appears at nearly the same spot on the horizon two weeks later on April 18, when it is nearly 16° lower left of Spica.

Here is our evening chart, with description following:

Evening Mid-twilight for April 2013

Evening mid-twilight in April:

In April 2013 at dusk, what seems to be the brightest “star” is actually Jupiter in the west, halfway from horizon to overhead early in month, descending to about a quarter of the way up in W to WNW at month’s end. Follow Jupiter from now until early June. It will lead you to a spectacular early evening twilight trio of Venus-Jupiter-Mercury, all fitting within a 5° binocular field very low in WNW during May 24-29. The gathering will be described and illustrated in next month’s issue of eCCS.

Next in brilliance after Jupiter at dusk in April is the star Sirius, descending in SSW to SW and twinkling noticeably. Technically ranking next is Canopus, still “visible” in southernmost California in April’s first week, but it’s so low that its apparent brightness is greatly dimmed. Therefore Arcturus is next in prominence after Sirius. Find this golden giant star very low in ENE on the 1st, ascending to one-third of the way up in the east by the 30th. After Spica appears in ESE, you can extend the curve of the Big Dipper’s handle to “Follow the arc to Arcturus, and drive a spike to Spica.”

The first-magnitude star 5-1/2° from Jupiter on April 1, widening to 9° below Jupiter by April 30, is Aldebaran, eye of Taurus the Bull. Orion’s bright shoulder, Betelgeuse, together with Sirius and Procyon, the bright stars of Canis Major and Canis Minor (the dogs following Orion across the sky), form the Winter Triangle, almost equilateral. North of that Triangle, nearly overhead at the start of April, are Pollux and Castor, 4-1/2° apart, marking the heads of the Twins of Gemini. (Castor isn’t plotted because it is of mag. +1.6.) Rigel, Orion’s bright foot, descends the western sky some 19° lower left of Betelgeuse. High in NW to upper right of Jupiter, is Capella, the “Mother Goat” star.

Note the huge oval of stars, in order, Sirius-Procyon-Pollux-(Castor)-Capella-Aldebaran-Rigel, and back to Sirius. This oval is sometimes called the Winter Hexagon, or Winter Ellipse. Betelgeuse is inside the figure, and from early April 2013 until mid-June 2014, Jupiter temporarily resides within the Hexagon. Regulus, heart of Leo the Lion, is the only bright star between the Winter Triangle and Arcturus, or between Pollux and Spica.

This month if we look outward, away from the Sun, we see two bright objects passing opposition to the Sun: first Spica on April 13, and Saturn two weeks later, on the night of April 27. At opposition, an object in or near the plane of our solar system appears about 180° from the Sun, rising around sunset, remaining visible all night, and setting around sunrise. You can use Spica or Saturn on their respective nights of opposition to accurately estimate the hour of the night!

Outdoor sky watching project: In May, Rigel, Aldebaran, Sirius, and Betelgeuse will disappear into the western evening twilight glow, and in June, Jupiter and the rest of the Hexagon will follow. Make a checklist of all the objects plotted on our April evening twilight sky map, and add Venus and Mercury to the list. Starting in mid-April, keep daily records of which objects you can see within an hour after sunset, and try to determine the first and last dates of visibility for each object.

The Moon graces the sky in evening mid-twilight during April 11-25, while waxing from a thin crescent low in WNW on April 11, to just past Full, low in ESE, on April 25. Follow the link to Sky Calendar www.pa.msu.edu/abrams/SkyCalendar/ for views of Moon passing planets and bright stars during the month, including Jupiter on April 14 and Saturn on April 25.

Here is our morning mid-twilight chart for April, showing sky from SoCal about 40-42 minutes before sunrise:

Morning Mid-twilight for April 2013

Look for Arcturus well up and descending in the west; Saturn with Spica preceding it, descending in SW to WSW; Antares, heart of the Scorpion, in the SSW; and the Summer Triangle of Vega-Altair-Deneb, nearing overhead. Sharp eyes in southern parts of our state might catch Mercury very low in E first half of April, and Fomalhaut rising in SE by month’s end.

Daytime viewing: The Moon is near Last Quarter phase (half full and 90° W of Sun) on Apr. 3 and May 2. So, April 1-4 and April 30-May 3 will be great school days for viewing the Moon with a telescope, as the first activity of the morning, especially if the telescope’s low-power eyepiece is fitted with a single polarizing filter, and the eyepiece is rotated in its tube to maximize the darkening of the blue sky.

Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs. 

Robert D. Miller did graduate work in Planetarium Science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.

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Written by Robert Victor

Robert Victor

Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs, CA. Robert is a member of CSTA.

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