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Jupiter remains a dazzling object this month and on April 7th the planet is at its best for the year, when opposition is reached. On this day, it will shine at mag. -2.5 and to the naked eye, the largest planet in the Solar System is unmistakable and easily brighter than any night-time star. It's currently located in Virgo, 4 degrees northeast of the constellation's brightest star, Spica (α Vir - mag. +1.0).
At opposition, Jupiter is visible all night long. The planet rises in the east at sunset, reaches its highest point in the sky around midnight before setting in the west at sunrise. It's best seen from equatorial regions, although from northern and southern temperate locations it still reaches a reasonable altitude during late evening and early morning.
Mercury reaches greatest eastern elongation (GEE) on April 1st, when it moves out to 19 degrees from the Sun. On this day from mid-latitude northern locations, the innermost planet will shine at mag. -0.2, 10 degrees above the western horizon, 45 minutes after sunset. It remains visible, although fading, until the second week of the month when eventually lost to the bright evening twilight. From mid-northern latitudes, this also happens to be the best evening apparition of the year. However, from southern locations, Mercury is unsuitably placed for observation during this time.
The diagram below shows the altitude of Mercury, 45 minutes after sunset as seen from latitude of 51.5N (e.g. London, England). A similar view exists at other northern temperate latitudes.
Algol (β Per) is a bright eclipsing binary system located in the northern constellation of Perseus and one of the best-known variable stars in the sky. Often referred to as the "Demon Star", most of the time it shines at mag. +2.1, but every two days, 20 hours and 49 minutes it suddenly dips in brightness to mag. +3.4, remaining dim for about 10 hours before returning to its original state.
Why the change in brightness? The Algol system consists of at least three-stars (β Per A, β Per B and β Per C) with the orbital plane of Algol A and B directly in line with the Earth. The regular dips in brightness occur when the dimmer B star moves in front of and eclipses the brighter A star. There is also an extra dimension in that a secondary eclipse occurs when the brighter star occults the fainter secondary, resulting in a very small dip in brightness that can be detected with photo-electrical equipment.
Mercury, the innermost planet, is now heading towards greatest eastern elongation (GEE), which it reaches on April 1, 2017. From mid-March, observers at northern-based locations should be able to spot the elusive planet, low down above the western horizon just after sunset. Each subsequent evening it improves in visibility until GEE is reached. After that, Mercury sinks gradually back towards the horizon until about 10 days later when it becomes lost to the bright twilight sky. This also happens to be the best evening apparition of the year from northern locations.
From southern latitudes, Mercury is unsuitably placed for observation this time.
NGC 6302, also known as the Bug Nebula or Butterfly Nebula, is a bipolar planetary nebula in the constellation of Scorpius. It's an unusual object, complex in structure with an incredibly hot star at its core. At a temperature of 250,000 degrees Celsius, this dying star is one of the hottest known in the galaxy. However, since it radiates predominantly in UV and is shrouded by dust, it's visually challenging to observe.
The discoverer of the Bug Nebula is debatable. Edward E. Barnard observed the planetary in 1880, although some references suggest that James Dunlop may have found it in 1826. The nebula itself shines at mag. of +9.6 and therefore well within the reach of medium size backyard scopes.
NGC 4559 is a spiral galaxy located in the constellation Coma Berenices. At mag. +10.0 it can be spotted with binoculars, but is challenging, requiring dark skies and patience. However, its high surface brightness does somewhat help. The galaxy is a nice telescope target that offers something for all sizes of backyard instruments.
NGC 4559 was discovered by William Herschel on April 11, 1785 and is estimated to be 30 million light-years distant. It's best seen from northern locations during the months of March, April and May.
NGC 3195 is a planetary nebula of mag. +11.5, located in the southern constellation of Chamaeleon. At a declination of -81 degrees, it's the closest bright planetary to the South Celestial Pole and therefore circumpolar from almost the entire Southern Hemisphere. Theoretically, it can also be seen from northern latitudes but only from south of +9 degrees. Even then, from such locations this planetary only scraps above the horizon and is an incredibly difficult, if not impossible, object to spot.
John Herschel discovered NGC 3195 on February 24, 1835. It's number 109 and the final object in the Caldwell catalogue.
Unfortunately, we haven't had a very bright comet for some time now, but there are a number of short-period comets that regularly approach the inner Solar System, and a few of them can even be spotted with binoculars. One of the more interesting is 41P/Tuttle-Giacobini-Kresak. It has an orbital period of 5.4 years and is currently heading back towards perihelion, which it reaches on April 12th. Over the coming months, this comet will be superbly placed in the evening sky for northern-based observers and bright enough to be seen with binoculars. It may even reach naked eye visibility.
Mercury passes through superior conjunction on March 7th, but it's not long before the fast moving planet moves out from the Sun and into the early evening sky. During the second half of the month, it re-appears low down towards the western horizon after sunset from northern locations. For example, on March 31st from London (51.5N) England, Mercury will be positioned 7.5 degrees above the horizon, one hour after sunset.
Between March 15th and 31st, observers should note that although the planets altitude improves slightly each subsequent evening, it brightness dims from mag. -1.5 to -0.4.
From southern temperate latitudes, Mercury is unsuitably placed for observation.
The current long evening apparition of Venus comes to an end this month. The brightest planet starts March as a dazzling beacon of light, visible after sunset above the western horizon. It shines at mag. -4.8, with an illuminated phase of 16 degrees and an apparent diameter of 47 arc seconds. As the month progresses, Venus falls back towards the Sun, fades in brightness and decreases in phase, but its apparent diameter improves to a maximum of 60 arc seconds. The apparition finally ends on March 25th, when inferior conjunction is reached. However, it's only about a week or so before Venus can be seen again, this time in the early morning sky.