What’s Up!

A monthly look at astronomical events in the sky and on Earth

March 2019

angles in the sky

The following article often refers to angles in the sky. The diagram above is a rough guide. Hold your hand at arm’s length. (Courtesy of timeanddate.com)

Fri/Sat/Sun 1st/2nd/3rd

A rare opportunity to catch a naked eye glimpse of elusive Mercury in the evening sky after sunset. Find a place where you’ve got a good view of the western horizon, note where the sun sets (at about 1740), wait until 1815 then look at a point 10° directly above where the sun set and there will be Mercury. Binoculars will help you to spot it initially but it’s easily visible without them. A small telescope will show it as a 36% illuminated orange coloured crescent. It’s visible until after 1845 as it slowly approaches the horizon.

BEWARE! Don’t try using binoculars to scan the sky for Mercury before the sun has completely set. You could blind yourself!

Sat 2nd

If you’ re awake by 0615, look low in the SE to see Venus and the crescent Moon together just 5° above the horizon. A nice sight.

Fri 8th

A wafer thin crescent Moon appears in the WSW after sunset (1754). It’s only 4% illuminated so will be a challenge to spot but it’s worth a try. By 1830 it should be easier to see, and Mercury may still be spotted at 5° altitude due west.

Mon 11th

The waxing crescent moon passes near to Mars. Spot the red planet 3° above and to the right of the Moon any time from 1900 to 2100 in the SW to W.

Thu 14th

Moon at first quarter. This is a great time to observe it, as the terminator is facing us and throwing lunar features into sharp relief.

Objects of the Month

There follows a shortlist of objects that are roughly in the south this month at 2030 hrs – a convenient time for family viewing.

Screenshot showing Gemini
Click on image to enlarge

The image above shows the sky at about this time, looking south (from Stellarium).

Castor and Pollux

The “heavenly twins” and the two brightest stars in the constellation of Gemini.

A 2 or 3-inch telescope will show Castor as a double star (separation 4 arc seconds) but in fact each of those stars is itself a double, discovered spectroscopically as they are too close to be separated optically. Each pair comprises a hot, bright A-class main sequence star orbited by a cool, faint M-type red dwarf. What’s more, this pair of doubles is orbited by another very faint double, making Castor an amazing 6-star system. Its combined magnitude is 1.58.

While your telescope is pointing at Gemini, take a moment to look at Pollux. Its orange colour is clearly very different from that of Castor. Pollux is a giant K-type star nine times the diameter of the Sun but only twice its mass. At 34 light years distant, it’s the nearest giant star to us. It does have a planet at least twice the size of Earth orbiting it called Thestias. Unfortunately, at only 1.6 astronomical units (a.u.) from the star when the habitable zone round Pollux is between  6 and 12 a.u., life on Thestias could be a tad challenging!

The Winter Triangle (Procyon, Sirius and Betelgeuse)

These three interesting stars form a near-equilateral triangle. See star map above.


Sirius double star
Hubble image (NASA)

SiriusThe brightest star in the sky. Is actually a binary. Sirius A is a main sequence A0 star; Sirius B is a white dwarf hundreds of times fainter than Sirius A. Although their angular separation is currently 10 arc seconds, the difference in brightness (magnitudes -1.44 and 8.5) makes spotting Sirius B a challenge for even large telescopes. The Hubble image on the left illustrates this.


Procyon A-B

Another binary star. Procyon A is an F5 star and thus cooler and yellower than Sirius. Procyon B is another white dwarf. Their magnitude difference and closer separation (4.3″) makes spotting the fainter component even more difficult than for Sirius but Giuseppe Donatiello has done it with a home made 127mm f/9 ED refractor!


Image courtesy of NASA

Now Betelgeuse is something quite extraordinary. It’s an M1 red supergiant and one of only a handful of stars that are large and near enough for specialist telescopes to see their surface as a disc. This is a NASA image. Put it where the sun is and it would extend nearly to the orbit of Jupiter. It’s a semiregular variable whose magnitude changes by 1.3; its surface temperature is only 3600K but it’s luminosity is 90,000 to 150,000 times that of the sun.

And finally, two Open Clusters, M41 and M47

M41, M47 clusters
Click image to enlarge

These often neglected clusters are fine targets in modest telescopes and even binoculars. M41 is 5° below Sirius, and M47 is 12° east and 2° north of Sirius. They’re both at magnitude 4.5 and about the apparent diameter of the full moon. Note that M41 contains a mixture of red giants and white dwarfs  but M47 comprises mainly young blue stars and only a few red giants.


Image courtesy of NASA


Image courtesy of NASA

Stop press – space news

As this item is being uploaded, we have news of the successful launch of Israel’s Beresheet lunar lander. This privately funded mission by Israel’s SpaceIL company lifted off using a SpaceX Falcon 9 rocket. The name Beresheet comes from the book of Genesis and means “In the beginning”. Clearly, Israel intends this to be the beginning of their efforts in space. For the latest news, see this Sky & Telescope article and this Beresheet fact sheet.

Clear skies and good viewing!

John Rowland