The Night Sky This Month: April 2018

Zodiacal Light
Image Credit: Zodiacal Light by Maureen Allen

This month offers some spectacular sky gazing opportunities, including throughout the whole of April the planets Mars and Saturn being seen low in the south-southeastern sky in the constellation of Sagittarius, and later joined by the Moon in a three way conjunction on the 7th. Other monthly highlights includes the Spring Triangle rising to prominence in the northern sky, marked by the brightest stars in three beautiful constellations, namely Arcturus in Boötes, Spica in Virgo, and Regulus in Leo.

Moon

The image above shows the Zodiacal Light, which is a diffuse glow of reflected sunlight that is sometimes seen in the night sky along the ecliptic, or the imaginary line that marks the annual path of the Sun, Moon and Zodiac constellations.

Sadly though, April offers some of the last opportunities during 2018 to observe the phenomenon, which also represents the remains of the accretion disc out of which the planets formed 4.5 billion years ago. Look for the cone-shaped dust ring that extends towards the south from the western horizon, but bear in mind that the Zodiacal Light is considerably fainter than the Milky Way, which might make it difficult to spot from severely light polluted areas.

Phases

Last Quarter: 8th
New Moon: 16th
First Quarter: 22nd
Full Moon: 30th

Planets

Mercury reaches a position of inferior conjunction with the Sun on the first day of April, but becomes a predawn object in the following days. However, Mercury never rises before about 40 minute before the Sun throughout the month, and remains very low on the horizon, which makes it difficult to observe at this time of the year.

Venus still dominates the early evening sky from its position in the constellation Aries. In fact, it is almost impossible not to see Venus as it is the brightest object in the sky until it sets, which is about 2.5 hours after sunset at month’s end. On the evening of the 17th, the Earth-lit waxing crescent Moon will approach Venus to within 6 degrees, so be sure to look out for this pairing.

Mars is still located in the constellation Sagittarius, and it will be brightening from magnitude +0.3 to reach magnitude -0.4 at month’s end. While it will not outshine Sirius, it will be noticeably brighter than Arcturus. On the 2nd of the month, Mars will be just one degree to the northward of the planet Saturn, which should make for an interesting study in contrasting colors; Mars shining with a distinctive ochre hue, while Saturn will be pale yellow with a magnitude of +1.12. On the first day of April, Mars will rise at about 02h00, but it will rise progressively earlier throughout the month, to rise at about 1h30 by month’s end, albeit low on the south-south-eastern horizon. On the 26th, Mars passes only 1.4 degrees below Pluto, which fact may or may not be of purely academic interest.

Jupiter rises at about 22h00 at the start of the month, and it will dominate the early evening sky from its position in Libra after Venus has set. As April wears on, Jupiter will set progressively earlier, until disappearing a few minutes after sunset at month’s end, making the King of the planets increasing difficult to spot. However, at this time of the year Jupiter forms a pretty triangle with the stars Zubenelgenubi and Zubeneschamali, the brightest stars in Libra, but note that Jupiter will outshine both stars by a considerable margin.

Saturn starts the month rising at about 02h00, to set soon after astronomical midnight by month’s end. As stated elsewhere, Saturn and Mars will be in close proximity, reaching conjunction on the 2nd, but note that the waning Moon will also be forming a close pairing with the pair of planets on the 7th. While Saturn remains close to the horizon, with a large telescope it might just be possible to spot its biggest moon, Titan, shining at magnitude +8.0 at its greatest western elongation from Saturn on the nights of the 4th and 20th, and its greatest eastern elongation from Saturn on the nights of the 12th and 26th. Note also that it might be possible to spot Titan to the south of Saturn on the 16th, and to the planets’ north on the nights of the 8th and 24th.

Uranus is still in the constellation Pisces, but since it reaches a position of conjunction with the Sun on the 18th, it is poorly placed for observation throughout the month.

Neptune is still in the constellation Aquarius, and although it rises at about 04h00 by the middle of the month, it is like Uranus, also not well placed for observation.

Meteor Showers

April sees two meteor showers, although neither shower is particularly prolific.

The Virginids is generally classified as a weak shower, and although it usually produces slow but long-lived meteors, no more than about 5 meteors per hour is expected to occur during the peak that occurs on the night of the 11th/12th of April.

The Lyrids meteor shower is associated with Thatcher’s Comet, and this year, up to about 10 meteors per hour are expected during the peak that occurs about two hours before dawn on the morning of the 22nd. Note that sporadic activity may be observed from a radiant close to the star Vega in the constellation Lyra for several days on either side of the peak date.

Deep-Sky Objects

Apart from the Ursa Major that is prominent near the zenith, other constellations that are prominent towards the south at this time include the eastern reaches of Hydra, Coma Berenices, Virgo, Boötes, and Corvus, all of which contain some interesting, if not spectacular deep sky objects (DSOs). Below are some details of a few such objects:

The Black Eye Galaxy (M64, NGC 4826)

M64
Image by jeffjastro.com

Also known as the Evil Eye Galaxy or Sleeping Beauty Galaxy, M64 is located about 24 million light years away in the constellation Come Berenices, and contain around 100 billion stars. It also has an apparent visual magnitude of 9.36, which makes it an easy target even for small telescopes. Look for M64 about one degree to the east-northeast of the star 35 Comae Berenices.

The galaxy’s outer regions spans an area 40,000 light years across, while its inner region spans 6,000 light years, and rotates at 300 kilometers per second. In scientific terms, this is a very interesting object, since the inner and outer regions of the galaxy rotate in opposite directions, and the region that separates the two contra-rotating parts is an active star-forming region. While the mechanisms that caused this strange behavior is not known with certainty, most astronomers believe that it was caused when M64 devoured a smaller galaxy roughly one billion years ago.

Boötes Void

Also known as the Supervoid, this empty region of space in the constellation Boötes spans about 250 million light years, and while it is not the biggest void known, it is one of the least densely populated ones, containing only about 60 or so galaxies. The area is so depopulated that one researcher, Gregory Scott Alderling, once stated in the 1980’s that “If the Milky Way had been in the center of the Boötes void, we wouldn’t have known there were other galaxies until the 1960s.”

NGC 5466

This globular cluster is located in the constellation Boötes, about 52,000 light years away from the galactic centre. NGC 5466 is notable for the facts that it is believed to be source of a stream of stars known as the “45 Degree Tidal Stream”, discovered in 2006, and which contains a horizontal branch population of blue stars that are all as metal-poor as “normal” globular clusters.

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