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.
Mercury then passes inferior conjunction on April 19th, after which it moves into the morning sky. Next month, the planet will be particularly well placed for observation from southern regions.
Venus is now an early morning object. At start of April, the brilliant planet rises in the east about an hour before sunrise, climbing higher each morning as the month progresses. However, it soon becomes much better placed from more southern latitudes. For example, from such locations it rises some three hours before the Sun on the 30th, compared to about half that time from further north.
Venus increases in brightness from mag. -4.2 to -4.7 during April. Its illuminated phase starts the month at just 2%, but increases to 26% by month's end. In the same time period, the apparent diameter of the planet's disk decreases from 58 to 38 arc seconds.
On April 23rd, the thin waning crescent Moon passes 5 degrees south of Venus.
Mars remains visible low down towards the west after sunset. The planet is now about as dim as it gets, shining at mag. +1.6. It starts the month in Aries, before crossing into Taurus on April 12th. During the third week of April, it passes just a few degrees from the Pleiades open cluster (M45) and on April 28th, the thin waxing crescent Moon passes 6 degrees south of Mars.
The highlight of the month occurs on April 7th, when Jupiter reaches opposition. On this day, the planet will be located 4.455 AU (667 million kilometers or 414 million miles) from Earth. The gas giant is currently moving retrograde in Virgo, four degrees northwest of Spica (α Vir - mag. +1.0). At mag. -2.5, Jupiter is unmistakable and easily brighter than any of the night-time stars. This month it's visible all night long, rising in the east at sunset, reaching its highest point in the sky around midnight, before setting in the west at sunrise.
Popular 7x50 or 10x50 binoculars reveal a very small pale coloured planetary disk. Also visible are the large Galilean moons (Io, Europa, Ganymede and Callisto) that continuously change position as they orbit the planet. Through small scopes, details such as the great equatorial belts and shadow transits of the moons can be observed. Larger scopes reveal the "Great Red Spot" and other cloud details, across the 44 arc second disk.
On April 10th, the almost full Moon passes 2 degrees north of Jupiter.
Saturn is located in Sagittarius and on April 6th reaches the first of its two stationary points for 2017. This date signals the change in the planets motion from direct to retrograde and is widely regarded as the beginning of this year's opposition period. By the end of April, Saturn rises about midnight for observers at northern temperate latitudes, but over 3 hours earlier for those located further south.
The planet is easy to find. It's located about 14 degrees northwest of the centre of the Teapot asterism of Sagittarius. As the month progresses, it brightens slightly from mag. +0.4 to +0.3, with its apparent size increasing marginally from 17 to 18 arc seconds. To the naked eye, Saturn appears creamy in colour. Of course the spectacular ring system is its most famous feature and even a small refracting telescope will show them. Through medium and large aperture scopes, they are a fantastic and breathtaking sight.
In addition, a handful of Saturn's moons are also visible. The largest and brightest is Titan, which shines at eight magnitude and can be seen with binoculars. Small scopes also reveal others, including Rhea, Tethys and Dione.
On April 16th, the waning gibbous Moon passes 3 degrees north of Saturn.
Uranus reaches solar conjunction on April 14th and is therefore positioned too close to the Sun to be safely observed this month.
Neptune, mag. +8.0, passed through solar conjunction at the beginning of last month. From northern temperate latitudes, the distant planet remains unsuitably placed for observation throughout April, but can be seen before sunrise towards the east from equatorial and southern regions. By months end from such locations, it rises up to four hours before the Sun.
Neptune is located in the faint constellation of Aquarius. Although never bright enough to be seen with the naked eye, it can be spotted with binoculars when the sky is dark enough. Currently positioned 15 degrees northeast of Neptune is much brighter, Venus.
On April 22nd, the thin waning crescent Moon passes 0.2 degrees north of Neptune, with an occultation visible from Australia (19:50 UT).
Solar System Data Table April 2017
|Date||Right Ascension||Declination||Apparent Magnitude||Apparent Size||Illum. (%)||Distance from Earth (AU)||Constellation|
|Sun||5th Apr 2017||00h 55m 51.7s||05d 58m 20.0s||-26.7||32.0'||100||1.000||Pisces|
|Sun||15th Apr 2017||01h 32m 35.0s||09d 40m 12.8s||-26.7||31.9'||100||1.003||Pisces|
|Sun||25th Apr 2017||02h 09m 53.7s||13d 06m 08.1s||-26.7||31.8'||100||1.006||Aries|
|Mercury||5th Apr 2017||01h 59m 45.5s||15d 23m 54.6s||+0.5||08.4"||28||0.798||Aries|
|Mercury||15th Apr 2017||02h 00m 38.7s||15d 11m 14.4s||+4.0||10.9"||04||0.616||Aries|
|Mercury||25th Apr 2017||01h 39m 17.5s||10d 40m 46.8s||+4.6||11.8"||02||0.570||Pisces|
|Venus||5th Apr 2017||23h 45m 08.9s||05d 43m 22.4s||-4.4||55.9"||05||0.298||Pisces|
|Venus||15th Apr 2017||23h 40m 37.9s||02d 49m 32.8s||-4.7||49.0"||13||0.340||Pisces|
|Venus||25th Apr 2017||23h 50m 15.0s||01d 32m 14.2s||-4.8||41.7"||22||0.400||Pisces|
|Mars||5th Apr 2017||03h 02m 50.1s||17d 40m 53.9s||+1.5||04.2"||97||2.254||Aries|
|Mars||15th Apr 2017||03h 31m 16.8s||19d 35m 28.4s||+1.5||04.1"||97||2.311||Taurus|
|Mars||25th Apr 2017||04h 00m 03.7s||21d 12m 05.8s||+1.6||04.0"||98||2.365||Taurus|
|Jupiter||5th Apr 2017||13h 10m 14.5s||-05d 44m 30.3s||-2.5||44.2"||100||4.457||Virgo|
|Jupiter||15th Apr 2017||13h 05m 28.9s||-05d 15m 31.3s||-2.5||44.2"||100||4.461||Virgo|
|Jupiter||25th Apr 2017||13h 00m 56.3s||-04d 48m 21.5s||-2.4||43.9"||100||4.494||Virgo|
|Saturn||5th Apr 2017||17h 49m 26.5s||-22d 04m 16.1s||+0.4||17.1"||100||9.706||Sagittarius|
|Saturn||15th Apr 2017||17h 49m 10.2s||-22d 03m 35.8s||+0.4||17.4"||100||9.552||Sagittarius|
|Saturn||25th Apr 2017||17h 48m 12.5s||-22d 02m 50.1s||+0.3||17.7"||100||9.412||Sagittarius|
|Uranus||5th Apr 2017||01h 28m 35.3s||08d 40m 40.2s||+5.9||03.4"||100||20.920||Pisces|
|Uranus||15th Apr 2017||01h 30m 44.2s||08d 53m 20.3s||+5.9||03.4"||100||20.933||Pisces|
|Uranus||25th Apr 2017||01h 32m 53.0s||09d 05m 53.9s||+5.9||03.4"||100||20.919||Pisces|
|Neptune||5th Apr 2017||22h 57m 33.5s||-07d 35m 02.4s||+8.0||02.2"||100||30.789||Aquarius|
|Neptune||15th Apr 2017||22h 58m 44.6s||-07d 28m 00.4s||+8.0||02.2"||100||30.688||Aquarius|
|Neptune||25th Apr 2017||22h 59m 48.1s||-07d 21m 47.3s||+7.9||02.2"||100||30.566||Aquarius|