Night sky highlights for February 2017

The night sky, due south, on February 4 2017.

The night sky, due south, on February 4 2017. - Credit: Archant

The Night Sky: Venus is unmissable in February, writes astronomer Lawrence Harris.

Several clear nights during January revealed the glorious spectacle of planet Venus shining in our evening skies. It will remain very bright during February while it slowly approaches Earth, right up to March 25, when it then moves invisibly between us and the sun to enter our morning skies.

I have been watching science-fiction films depicting planets Mercury and Venus and sadly had to abandon them because the science was so awful it ruined credibility for me! One astronomer was shown supposedly looking at Mercury with his telescope pointing high in the sky late at night - but Mercury, of course, is never more than a very few degrees above the horizon during twilight.

It surprises me that such obvious errors should be ignored by the producers. Of course I could also mention the complete lack of gravity often simply ignored by film makers when depicting astronauts travelling in deep space, who walk around as if in a film studio. Star Trek does make the effort to explain these effects so I am a fan. So do you have a favourite sci-fi gaffe?


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February's planets

Many are on view this month. Mars appears near Venus in the western sky for some time while they both change their positions daily. Venus vanishes into the sunset later in March. Meanwhile Jupiter rises in the east before midnight and is spectacular even in binoculars. Saturn rises in the east just before morning twilight.

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On Sunday evening February 26, point your binoculars at Venus and then at Mars. Close to Mars you may see the turquoise disc of the planet Uranus. It is easily bright enough for binoculars and should be identifiable.

Occultations galore

During the Moon's monthly orbit around Earth we see its various phases during clear nights. Tonight (February 4) sees the quarter moon approaching the constellation of Taurus - see star chart. Tomorrow evening we have an excellent opportunity to see the Moon crossing these bright stars and occulting (hiding) several. Because the Moon is so close to Earth (about one-quarter of a million miles away) it appears to move rapidly eastwards in front of the starry background. When it crosses a star-field the effect can be quite dramatic.

A pair of binoculars will show the Moon surrounded by bright stars after sunset tomorrow as it glides within Taurus. The beauty of the Moon's phase can now be seen; the dark part of the Moon may be shining faintly due to earthshine (sunlight reflected off the Earth's clouds on to the Moon) enabling the dark limb of the Moon to be seen.

As you watch this limb you may notice its slow motion eastwards. This motion will take it in front of the stars, and when it happens you will see the sudden cut-off of the starlight. With a bright star the effect is very dramatic and shows how fast the Moon is really moving. After dark, watch these multiple occultations and enjoy the drama. And remember to view the nearby beautiful Pleiades star cluster too.

February's penumbral eclipse of the Moon

The sun causes a shadow whenever anything gets between it and something else. If you stand in your garden on a sunny day, your shadow is cast across the garden. The Earth does the same thing on a gigantic scale. As it orbits the sun, taking a year to do so, Earth's shadow is cast into space, following behind as a huge cone of darkness. The cone actually consists of two parts: the inner dark part (called the umbra) where no sunlight gets through, and the outer, larger cone (called the penumbra) where some sunlight gets through. If the Moon passes through the inner part we see an umbral eclipse; if it passes through the outer part we see a penumbral eclipse.

On almost every occasion around new moon, the Moon passes below or above the sun moving from right to left. If it passes directly in front, we have a solar eclipse; these are extremely rare because the Moon itself is so small. Similarly, around full moon, the Moon usually passes above or below the Earth's gigantic cone of darkness. When it passes directly behind Earth, we have a lunar eclipse - but these are not so rare because the cone of the Earth's dark shadow is huge so there is a much bigger chance of the Moon entering it, or at least passing close.

During the late evening of Friday February 10 the Moon will be approaching the Earth's outer cone of darkness (the penumbra) from the right-hand side and below. The Moon is then behind the Earth and moving anticlockwise. At 10.34pm the Moon's upper left edge makes its first contact with the outer (penumbral) cone. It continues to move eastwards, moving deeper into the penumbra. Its whole passage is within the penumbra so this is a penumbral eclipse.

It won't therefore be as dark or dramatic as a total eclipse of the Moon but it may have a significant effect on the Moon's brightness because at the time of greatest coverage at 00.34am on Saturday February 11, it will almost touch the umbra, being 99pc inside the penumbra. At this time there may be a dark region near the upper right of the Moon's disc. I ran a simulation of the eclipse using Stellarium - a free astronomy program that runs on several platforms including Windows. It allows me to look ahead to get a good impression of forthcoming eclipses. The Moon will be between the constellations of Leo and Cancer and much dimmer than usual so you may see the brighter stars nearby.

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