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Mercury

Mercury passes inferior conjunction on April 1st, but it doesn't take long before the planet rises out of the Sun's glare. It should be visible, low down above the eastern horizon before sunrise from tropical and southern latitudes by about the middle of the month. The planet subsequently brightens and climbs higher in the sky each morning until it reaches a peak altitude on April 29th, the date of greatest elongation west (27 degrees from the Sun). On this day, from latitude 35S (approx. equal to Sydney, Cape Town and Santiago), Mercury shines at mag. +0.3 and appears 17 degrees above the horizon, 45 minutes before sunrise. The very thin waning crescent Moon passes 4 degrees south of the planet on April 14th. On the 23rd, Mercury reaches aphelion when it's 0.467 AU (approx. 69.9 million kilometres or 43.4 million miles) distant from the Sun.

From northern temperate latitudes, this is not a great apparition due to the shallow angle of the ecliptic to the horizon and even at greatest elongation the planet appears low down. Observers will struggle to easily spot it.

Venus

Venus, mag. -3.9, is now a brilliant evening star that can be seen above the western horizon just after sunset. From northern temperature latitudes the planet sets about 2 hours after the Sun, although the visibility period is up to an hour less for those located further south.

During April, the apparent size of Venus increases from 10.6 to 11.5 arc seconds, with its illuminated disk decreasing from 94% to 89%. On April 17th, the thin waxing crescent Moon passes 5 degrees south of Venus, providing pleasant early evening viewing.

Venus as an evening star during April 2018 (credit:- freestarcharts)

Mars

Mars is in Sagittarius and starts the month just southwest of Saturn. However, it doesn't take long before the red plant moves away from its more distant Solar System neighbour, and by months end the pair are separated by some 15 degrees. Mars is now starting to brighten nicely. On April 1st, the planet shines at magnitude +0.3, by month's end it's up to magnitude -0.3. During the same period, its apparent size increases from 8.5 to 11.0 arc seconds. At medium to high magnifications, it's now possible to make out the main surface details with a telescope, such as the Syrtis Major. For comparison, Saturn improves from magnitude +0.5 to +0.3 during this time, although it's apparent diameter is much larger at 17 arc seconds.

Mars, like Jupiter and Saturn, is currently better placed for observation from southern and tropical locations. From such locations, the planet rises just after midnight at the start of the month, improving to late evening by months end. From mid-latitude northern latitudes it appears much lower down and has a shorter visibility period. At months end, it rises during the early hours of the morning.

On April 7th, the waning crescent Moon passes 3 degrees north of Mars. Mars passes 1.4 degrees south of dwarf planet Pluto on April 26th. At magnitude +14.3, Pluto is a target for observers with large scopes. An instrument of at least 300mm (12-inch) aperture is recommended for this task.

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Mars and Saturn during April 2018 (credit:- freestarcharts)

Mars and Saturn during April 2018 - pdf format (credit:- freestarcharts)

Jupiter

Jupiter is now a brilliant object as it heads towards opposition in May. Throughout the month the gas giant continues to move retrograde amongst the faint stars of Libra. Jupiter is currently better seen from southern and equatorial latitudes, where it appears higher in the sky, compared to northern temperate locations. At the start of April, it rises mid-evening from tropical and southern locations and around midnight from mid-latitude northern temperate locations. By month's end, it's practically visible all night.

This month, Jupiter's magnitude brightens slightly from mag. -2.4 to -2.5 with its apparent diameter increasing from 42.6 to 44.6 arc seconds. The planet easily outshines all night-time stars. With binoculars or small telescopes up to four Galilean moons are visible (Io, Europa, Ganymede, Callisto). Telescopically, Jupiter is a gem and even small refractors will show much detail, including the main cloud belts. Medium and larger size scopes reveal considerably more detail, such as smaller belts, ovals, festoons and the well-known, but now shrinking, Great Red Spot.

On April 3rd and 30th, the waning gibbous Moon passes 4 degrees north of Jupiter.

Jupiter during April 2018 (credit:- freestarcharts)

Jupiter during April 2018 - pdf format (credit:- freestarcharts)

Saturn

Saturn is in Sagittarius and begins the month at magnitude +0.5, just over a degree northeast of slightly brighter Mars (mag. +0.3). The colour difference of the two planets is readily noticeable. Mars appears distinctly orange with Saturn creamy or off-white. As previously mentioned, it doesn't take long before Mars races off to the east and by month's end the two planets are separated by some 15 degrees.

On April 7th, the waning crescent Moon passes 2 degrees north of Saturn. On April 16th, Saturn reaches aphelion when it's 10.066 AU (approx. 1,505.9 million kilometres or 935.69 million miles) distant from the Sun. On the following day, the beautiful ringed planet reaches the first of its two stationary points. This date signals the change in apparent motion from direct to retrograde, and is widely regarded as the beginning of this year's opposition period. By the end of April, Saturn a couple of hours after midnight for observers at northern temperate latitudes, but up to 4 hours earlier for those located further south.

When seen through a telescope, Saturn is a wonderful sight. A small 80mm (3.1-inch) refractor easily reveals the ring system, which is currently wide open at a tilt of 26 degrees to the line of sight. A 150mm (6-inch) or 200mm (8-inch) reflector shows a wealth of details, such as planetary cloud formations, ring divisions and up to half a dozen satellites. During the month, the apparent diameter of Saturn's disk increases slightly from 16.7 to 17.5 arc seconds. On April 30th, it will shine at magnitude +0.3.

Moon, Mars, Jupiter and Saturn from northern locations on the morning of April 7th (credit:- freestarcharts/stellarium)

Uranus

Uranus reaches solar conjunction on April 18th and is therefore positioned too close to the Sun to be safely observed this month.

Neptune

Neptune, mag. +8.0, is located in the faint Aquarius and 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.

Although never bright enough to be seen with the naked eye, Neptune can be spotted with binoculars under dark skies. On April 12th, the thin waning crescent Moon passes 2 degrees south of Neptune.

Solar System Data Table - April 2018

 DateRight AscensionDeclinationMag.App. SizeIllum. (%)Dist. (AU)Constellation
SunApr 0100h 40m 19.6s04d 20m 22.1s-26.732.0'1000.999Pisces
SunApr 1501h 31m 38.4s09d 34m 42.2s-26.731.9'1001.003Pisces
SunApr 3002h 27m 51.9s14d 36m 39.1s-26.731.8'1001.007Aries
MercuryApr 0100h 40m 37.2s07d 35m 46.0s6.511.2"10.601Pisces
MercuryApr 1500h 17m 25.6s01d 21m 35.1s1.710.5"170.644Pisces
MercuryApr 3000h 50m 17.2s02d 20m 23.1s0.38.0"440.843Cetus
VenusApr 0101h 55m 08.2s11d 16m 46.5s-3.910.6"941.578Aries
VenusApr 1503h 01m 41.8s17d 17m 39.5s-3.910.9"921.524Aries
VenusApr 3004h 16m 41.5s22d 08m 17.9s-3.911.5"891.455Taurus
MarsApr 0118h 34m 01.5s-23d 33m 05.5s0.38.5"881.108Sagittarius
MarsApr 1519h 06m 43.7s-23d 17m 48.2s0.09.5"880.982Sagittarius
MarsApr 3019h 39m 01.2s-22d 46m 32.4s-0.311.0"880.853Sagittarius
JupiterApr 0115h 20m 21.3s-17d 05m 45.0s-2.442.6"1004.625Libra
JupiterApr 1515h 15m 29.4s-16d 45m 56.2s-2.443.8"1004.496Libra
JupiterApr 3015h 08m 35.3s-16d 18m 20.0s-2.544.6"1004.416Libra
SaturnApr 0118h 37m 26.6s-22d 16m 26.6s0.516.7"1009.976Sagittarius
SaturnApr 1518h 38m 26.2s-22d 15m 20.9s0.417.1"1009.745Sagittarius
SaturnApr 3018h 37m 57.8s-22d 15m 40.0s0.317.5"1009.516Sagittarius
UranusApr 0101h 41m 54.5s09d 59m 48.8s5.93.4"10020.850Pisces
UranusApr 1501h 44m 54.7s10d 16m 58.1s5.93.4"10020.893Pisces
UranusApr 3001h 48m 10.1s10d 35m 19.0s5.93.4"10020.879Aries
NeptuneApr 0123h 05m 03.5s-06d 52m 02.9s8.02.2"10030.836Aquarius
NeptuneApr 1523h 06m 46.8s-06d 41m 40.4s8.02.2"10030.709Aquarius
NeptuneApr 3023h 08m 21.6s-06d 32m 17.1s7.92.2"10030.527Aquarius