Last month Mercury reached greatest eastern elongation of the Sun and for a few days was an easy evening target for the northern hemisphere, but not well placed for southern hemisphere observers. This month the roles are reversed. The elusive planet, which moves so fast that it orbits the Sun once every 88 days, recently passed inferior conjunction and reaches greatest elongation west on April 18th (27 degrees).
For southern hemisphere sky watchers this is the best morning display of the year. For most of the month, observers will be able to follow Mercury in the eastern sky before sunrise. At greatest elongation on the April 18th, Mercury rises two hours before the Sun and at magnitude 0.3 should stand out against the faint background stars of Pisces. A thin crescent Moon will be nearby. On April 23rd, look 2 degrees to the north of Mercury with binoculars and you may be able to spot magnitude 5.9 Uranus.
The elongation is unfavourable for mid-latitude northern hemisphere observers and Mercury is not readily observable.
The early evening sky this month still belongs to Venus. Following on from last month's greatest eastern elongation, the brightest of all planets is still a dazzling object positioned above the western horizon after sunset. Starting April at magnitude -4.5 and ending the month at magnitude -4.7, Venus increases in brightness and on April 3rd the planet enters and then makes a spectacular bypass of the famous Pleiades (M45) star cluster. It is well worth the effort to observe such a rare event; it will not be until 2020 when this conjunction reoccurs and even with just a pair of binoculars the observer will be treated to an outstanding view.
At the start of April, for mid latitude northern hemisphere observers, Venus sets over 4 hours after the Sun. By months end little has changed, with Venus still setting 3.5 hours after sunset. For the mid latitude southern hemisphere observers, Venus sets about 2 hours after the Sun during April.
During the month the phase of Venus decreases from 48% to 27% and the apparent diameter increases from 25 to 37 arc seconds.
Having only last month reached opposition, Mars remains well placed for observation this month. The red planet is located in Leo and during the first half of the month it moves slowly, continuing its westerly retrograde motion until it reaches its stationary position on April 15th. Mars will then be less than 5 degrees from Leo's brightest star, magnitude 1.3 Regulus, before it ends its retrograde arc and continues to head eastwards again.
Mars is visible after sunset and for most of the night during April, however the brightness will notably decrease as the month progresses. On April 1st, Mars shines at magnitude -0.7 but by April 30th it is at magnitude 0.0. For telescope observers and images, the apparent diameter of the planet is shrinking (from 12 to 10 arcseconds) so surface details will start to become a little more difficult to record.
Jupiter is now a dusk object in Aries that is best seen during the first half of the month. The giant planet shines at mag -2.0 but it won't be long before it is lost in the Sun's glare. On April 22nd a thin crescent Moon will pass Jupiter.
It's the best time of the year to view the ringed planet as on April 15th, Saturn reaches opposition (8.72 AU or 1304 million km from Earth) and is visible all night. The brightness of Saturn varies little during the month, starting at magnitude 0.3, peaking at magnitude 0.2 at opposition and back to magnitude 0.3 at months end. The planet spends the month in Virgo, moves little and is located less than 6 degrees from the constellations brightest star, Spica (mag. 1.0). To the naked eye Saturn is noticeably the brighter of the two with a yellowish hint compared to the white/blue Spica.
With an apparent diameter of 19 arc seconds and a ring tilt of 14 degrees, Saturn is a spectacular sight through telescopes. Small telescopes will easily show the rings and the brightest moons, while large telescopes bring out more subtle details on the planets disk and rings. It is also a good time to look for the Cassini division, the main gap between the rings.
On April 7th, Titan the largest and brightest moon of Saturn reaches greatest western elongation and at magnitude 8.4 is a binocular object.
Uranus is located in Pisces, was at solar conjunction last month and is still to close to the Sun to be easily observed this month. Southern hemisphere observers have a chance to spot the planet in the morning sky shortly before sunrise on April 23rd, when it will be 2 degrees north of Mercury (magnitude 0.2). With binoculars you may be able to spot Uranus (magnitude 5.9).
Neptune is not well paced for observation for northern hemisphere observers this month. For southern hemisphere Neptune is a morning object, visible before sunrise during the second half of the month. Neptune is located in Aquarius and at magnitude 7.9 requires at least a pair of binoculars to be seen.
Solar System Data Table - April 2012
| ||Date||Right Ascension||Declination||Mag.||Size||Illum. (%)||Distance (AU)||Constellation|
|Sun||5th April 2012||00h 57m 31.0s||06d 08m 32.4s||-26.7||32.0'||100||1.000||Pisces|
|Sun||15th April 2012||01h 34m 16.1s||09d 49m 51.6s||-26.7||31.9'||100||1.003||Pisces|
|Sun||25th April 2012||02h 11m 37.6s||13d 14m 59.5s||-26.7||31.8'||100||1.006||Aries|
|Mercury||5th April 2012||23h 37m 31.5s||-02d 28m 21.7s||1.3||10.0"||21||0.670||Pisces|
|Mercury||15th April 2012||23h 57m 04.6s||-02d 26m 32.6s||0.4||8.4"||40||0.801||Pisces|
|Mercury||25th April 2012||00h 36m 26.8s||00d 53m 43.1s||0.1||7.1"||56||0.951||Cetus|
|Venus||5th April 2012||03h 53m 08.6s||24d 03m 40.8s||-4.6||26.0"||46||0.642||Taurus|
|Venus||15th April 2012||04h 29m 43.2s||26d 10m 33.6s||-4.6||29.6"||40||0.564||Taurus|
|Venus||25th April 2012||05h 01m 04.2s||27d 25m 16.7s||-4.7||34.1"||32||0.489||Taurus|
|Mars||5th April 2012||10h 29m 02.7s||12d 53m 16.2s||-0.6||12.2"||96||0.766||Leo|
|Mars||15th April 2012||10h 26m 24.3s||12d 40m 55.4s||-0.4||11.3"||94||0.828||Leo|
|Mars||25th April 2012||10h 28m 24.8s||12d 03m 01.2s||-0.2||10.4"||92||0.898||Leo|
|Jupiter||5th April 2012||02h 48m 01.4s||15d 16m 23.6s||-2.1||33.7"||100||5.846||Aries|
|Jupiter||15th April 2012||02h 57m 00.9s||15d 57m 06.3s||-2.0||33.3"||100||5.916||Aries|
|Jupiter||25th April 2012||03h 06m 17.6s||16d 37m 04.0s||-2.0||33.0"||100||5.967||Aries|
|Saturn||5th April 2012||13h 44m 14.2s||-07d 49m 20.1s||0.3||19.0"||100||8.737||Virgo|
|Saturn||15th April 2012||13h 41m 23.2s||-07d 32m 40.3s||0.2||19.1"||100||8.720||Virgo|
|Saturn||25th April 2012||13h 38m 30.7s||-07d 16m 22.3s||0.3||19.0"||100||8.733||Virgo|
|Uranus||5th April 2012||00h 20m 00.4s||01d 24m 49.2s||5.9||3.3"||100||21.055||Pisces|
|Uranus||15th April 2012||00h 22m 03.0s||01d 37m 59.0s||5.9||3.4"||100||21.013||Pisces|
|Uranus||25th April 2012||00h 24m 00.5s||01d 50m 31.1s||5.9||3.4"||100||20.945||Pisces|
|Neptune||5th April 2012||22h 17m 31.8s||-11d 13m 01.8s||8.0||2.2"||100||30.718||Aquarius|
|Neptune||15th April 2012||22h 18m 35.5s||-11d 07m 16.4s||7.9||2.2"||100||30.593||Aquarius|
|Neptune||25th April 2012||22h 19m 29.7s||-11d 02m 25.5s||7.9||2.2"||100||30.450||Aquarius|