The Night Sky This Month: April 2017

Night Sky April 2017
Image Credit: moonwise.co.uk

As the winter constellations gradually start disappearing off the western horizon, their spring counterparts have started rising to prominence in the night sky. Three such constellations can easily be observed in the northern sky during the mid-evening hours by locating the Spring Triangle, a beautiful asterism consisting of the brightest stars in Boötes, Virgo and Leo. Furthermore, this month it will have a royal visitor in the guise of the King of Planets, Jupiter, while towards the western part of the sky can be seen the red planet Mars in the constellation of Taurus.

Moon

First Quarter: 3rd
Full Moon: 11th
Last Quarter: 19th
New Moon: 26th

The Moon will present itself as an excellent photographic opportunity all through the night on the 10th of April, when it will pass a mere two degrees or so above Jupiter, now in the constellation Virgo. In addition, just before dusk on the 28th of the month, the Moon will occult the star Aldebaran in Taurus. The star will disappear behind the Moon at 20:07 BST (as seen from London), and reappear from behind the Moons’ bright limb at 20:07 BST, also as seen from London. Aldebaran will be visible just below the Moon as darkness falls.

Constellations

The map above shows the prominent constellations that are visible from the northern hemisphere at this time of year. This group includes a procession of zodiac constellations led by Taurus, which is now starting to set in the south-west around twilight, followed by Gemini, Cancer and Leo towards the south, and then Virgo, the Maiden. The brightest stars in Leo and Virgo, which are Regulus and Spica respectively, form part of the Spring Triangle, together with Arcturus in Boötes. Also prominent at this time of year is the constellation Ursa Major, which is almost overhead in the northeast around 10pm.

Planets

– Mercury will be at an elevation of about 14 degrees above the western horizon at nightfall on the first day of the month. On this date, the little planet will be at its greatest elongation from the Sun, about 19 degrees, and shining at magnitude -0.2. However, it will dim to about magnitude +3.0 by the 18th of the month as it drops back towards the Sun, to reach a position of inferior conjunction on the 20th of April. By month’s end, Mercury will reappear as a bright pre-dawn object, but despite its angular diameter increasing from 7.5 seconds of arc on the 1st to about 11 seconds of arc by the 18th, no surface detail will be discernible.

– Venus starts the month rising in the east around an hour or so before dawn, to climb to about 13 degrees above the horizon at dawn by month’s end. At the beginning of April, Venus will show a thin crescent about 1 minute of arc tall, with only about 2% of the planet’s disc being illuminated; by month’s end, Venus will shine at magnitude -4.7, having an angular diameter of 39 seconds of arc. At this time, about 26% of the planet’s disc will be illuminated, and keen observers of Venus will also be able to image the planet in infrared during the daylight hours.

– Mars starts the month in Aries, but by the 12th, the Red planet will have moved into Taurus. Look for Mars at about 20 degrees above the western horizon during the first days of April at sunset. Mars will however lose some altitude as the month wears on, to end the month only about 11 degrees or so above the horizon, which could make spotting it difficult. Mars will also dim somewhat as the month progresses, from about magnitude +1.5 to +1.6, as its angular diameter shrinks from 4.2 seconds of arc to 3.9 seconds of arc. Nonetheless, on the 25th of the month, Mars will be caught between the Pleiades and Hyades star clusters, about 9 degrees to the right of the star Aldebaran, which should compensate for the fact that no surface detail will be visible on the planet during April.

– Jupiter is visible for most of the night during April, as it comes into opposition on the 7th of the month. Look for the King of the planets due south at about 34 degrees on this date. Jupiter’s brightness will remain fairly constant during the month, reducing from magnitude -2.5 to -2.4, as its angular diameter shrinks slightly from 44.2 seconds of arc, to 43.6 seconds of arc. The month of April also offers several excellent opportunities to observe the Great Red Spot, which should be very close to the central meridian at around 22:01 on April 1st and around 20:53 by the month’s end.

– Saturn is now in the western reaches of the constellation Sagittarius, and rises at about midnight, to reach its highest point just before dawn. However, even though the planet’s brightness increases somewhat from magnitude +0.4 to +0.3, and its angular diameter grows from 17 seconds of arc to 18 seconds of arc as the month wears on, it remains a difficult planet to observe since it never rises more than about 18 degrees above the horizon for the remainder of 2017. Observers in mid-northern latitudes may therefore find it difficult to observe the ring system through the murk of the atmosphere, which is a perfectly good reason to splurge on an atmospheric dispersion corrector with which to tease decent views of Saturn’s ring system out of the thick atmosphere low above the horizon.

Meteor Showers

– The Lyrids is considered to be oldest known meteor shower, and the only meteor shower occurring during April. Associated with the comet Thatcher that takes around 415 years to orbit the Sun, this shower usually peaks on the 22nd of April. Although the usual maximum rate rarely exceeds about 10 to 15 meteors per hour in the hours between midnight and dawn, the Lyrids sometimes produces “bursts” of meteors that can exceed 100 or so per hour. Radiating from a point close to the star Vega in the constellation Lyra, observers in the northern hemisphere, as well as observers in mid-southern latitudes should have a clear view of the Lyrids as there will only be thin crescent Moon in the sky during the hours just before dawn.

Deep Sky Objects

April will provide the opportunity to locate many easy deep-sky targets using binoculars and small telescopes, including two stunning galaxies in the constellation of Ursa Major:

Bode’s Galaxy– Bode’s Galaxy (Messier 81, M81, NGC 3031) located about 11.8 million light years away, is the largest member of the 34-member M81 group of galaxies, one of several groups of galaxies in Ursa Major. M81 is so bright (magnitude 6.94) that is among the easiest of galaxies to observe with amateur equipment, so look for M81 about 10 or so degrees to the northwestward of the star Dubhe, also known as Alpha Ursae Majoris.

Pinwheel Galaxy– M101 (Pinwheel Galaxy), roughly equal in size to the Milky Way, is located about 25 million light years away, and at magnitude 7.86, is bright enough to be an easy binocular target, although it is not visible to the naked eye. The asymmetrical shape of the Pinwheel is caused by tidal interactions with nearby companion galaxies, which interactions are compressing vast amounts of interstellar hydrogen, thus creating large numbers of stars in the spiral arms of the galaxy in the process.

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