The Night Sky this Month

February 2010 by Russell Eberst

Although February is the shortest month, thanks to a couple of Roman Emperors, it is also the month that has the most impressive evening sky. As the sky gets dark in February, the prominent constellation of Orion will be among the first to become visible. Orion is surrounded by a host of other easily-recognised constellations with many bright stars included. So let's consider some of the objects on show.

Orion consists of a quadrilateral of bright stars with Betelgeuse and Rigel at two of the corners of the figure that marks the 'mighty hunter'. The redness of Betelgeuse is very obvious, especially when compared with the bluish-white of Rigel. Midway between these two can be found the three stars that make up the belt of Orion. These have the Greek letter designations of delta, epsilon, and zeta Orionis.

The line that they form can be extended to the north-west to reach Aldebaran in Taurus, which appears to be part of the V-shaped Hyades cluster. Further to the north-west is the more compact Pleiades cluster, also known as the 'seven sisters'. If we extend the line of Orion's belt to the south-east, we reach Sirius in Canis Major.

To the north and east of Orion are found more bright stars, primarily Procyon in Canis Minor, Castor and Pollux in Gemini and Capella in Auriga. The different brightnesses of stars are measured on a scale of 'magnitude'. The brighter a star is, the smaller its magnitude. So stars such as Pollux and Aldebaran are listed as first-magnitude, and the stars of Orion's belt as second magnitude. This continues down to the sixth magnitude, often quoted to be the limit for observers using the unaided eye.

The scale is extended to zero-magnitude stars like Capella, and to the brightest of all stars in our sky, Sirius, which is around magnitude -1.4. Within striking distance of Gemini is the prominent planet Mars, which is actually located in the neighbouring zodiacal constellation of Cancer.

The red planet, Mars, reached opposition at the end of January, and is well positioned for observation. It rises just before sunset, and is visible for most of the night. At magnitude -1.2, it rivals Sirius in brightness. The appearances of Sirius and Mars are somewhat different. Mars has an obvious red colour (though some describe it as 'ochre'), and is very steady in brightness, typical of a planet.

Sirus, or the Dog Star, shows the characteristic stellar rapid fluctuations referred to as 'twinkling'. These prominent variations are caused by atmospheric air currents in the lower layers of the Earth's atmosphere, and are in no way associated with the star itself. For bright stars such as Sirius, the atmosphere can also give rise to flashes of colour such as red, blue and green.

Through the month, Mars will steadily fade, losing nearly half its initial intensity, to end February at magnitude -0.6. Nevertheless, it will still outshine all stars in our night sky with the exception of Sirius. Mars is retrograding (moving westwards), being north of gamma Cancri by around 1° on February 4, and about 2°.5 north of the open cluster M44 (the beehive, or Praesepe).

This approach is one of the more distant oppositions of Mars, as will again be the case at its next opposition on March 3, 2012. Of the other planets, only Saturn is reasonably placed, but is mainly a morning object. It rises around 21.40 at the beginning of February, and two hours earlier at the end. Saturn reaches its highest elevation when it crosses the meridian, well after midnight. Its ring system is presented to us at a small angle which means details are hard to discern.

Jupiter and Venus are in conjunction on February 16, but are only 9° from the Sun, so will be lost in its glare. Mercury too, is poorly placed for observation in February. In compensation, the asteroid Vesta, brightest of all minor planets, reaches a favourable opposition, and can be easily found in Leo, especially around February 18, when it passes between gamma Leonis (Algieba) and 40 Leonis, which is less than 0°.4 away. Algieba is part of the famous 'sickle' or backwards question mark that makes up the head and heart of the Lion.

Vesta is magnitude +6.1, so readily spotted in binoculars. Its distance is 131 million miles, or 211 million km. A much smaller asteroid is Apophis, which is scheduled to make a very close approach to Earth on 2029 April 13. However, this February it makes a much more distant approach, but nevertheless will be actively tracked to refine its orbit, making future predictions more reliable.

Among the space launches scheduled for February, we have the Endeavour shuttle on another construction visit (STS130) to the International Space Station, launching on February 7, and Cryosat 2 on February 25. It is hoped that the launch from the Baikonur site will be more successful than Cryosat 1 which failed to reach orbit in October 2005.

February is the month most favoured for the appearance of rare types of cloud known as nacreous or 'mother-of-pearl'. Although not strictly an astronomical phenomenon, it can provide a spectacularly beautiful sight for sky-watchers. A flood of very cold polar air is thrust into the layers of the atmosphere between 8 and 12 km high, and gives rise to very colourful and impressive clouds that are seen at their most vivid around the time of sunset. Observers should be alert on all clear days, and keep the sky under regular scrutiny.


Moon phases

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