Archive for the ‘Moon’ Category

Blue Moon

Posted: August 1, 2012 in Astrobites, Moon

It’s August 2nd and there’s a full moon in the sky!

That’s an old picture; unfortunately it’s raining so I can’t see the moon right now.  Fortunately, this month we get two full moons, instead of the usual one!

The time between two successive full moons is about 29.5 days – give or take a little.  This means that in most calendar months there can only be one full moon.  However, if a full moon falls within the first day or two of the month then it’s possible for another full moon to fall before the end of the same month.

That’s exactly what’s happening in August; there will be full moons on August 2nd and 31st.  The second full moon on the 31st is called a blue moon…

In the 19th century the Maine Farmer’s Almanac divided the year into quarters and listed the dates of the full moons.  Obviously, the full moon provided much needed illumination for farmers after sunset so they were important.  Normally there’d be three full moons in each quarter of the year, but occasionally there is an extra full moon would fall in a quarter.  Farmers always called the final moon of the quarter the “late moon” and so the third full moon was deemed to be the extra one; they called it a blue moon.  In many cultures the usual pattern of monthly full moons had names like “Grain moon” (August) or “Harvest moon” (September).  The extra full moon which infrequently appeared in the calendar was known as a “betrayer” moon or a blue moon.  The phrase “once in a blue moon”…well, perhaps it’s from this phenomenon.  They aren’t actually that rare; blue moons will occur once every two and half years on average.

One final note; the term blue moon doesn’t refer to the actual colour of the moon.  The moon usually looks white, yellow or orange-red depending on how high it is in the sky (because of how much air the moonlight has to pass through).  However, volcanic eruptions or forest fires can inject just the right size of dust particle into the atmosphere to give the moon (and Sun) a bluish cast.  These kinds of events are also comparatively rare, happening perhaps less often than a blue moon!


Here’s a brief run down of where the planets are going to be in July.  Check out my planetary elongation chart to see when conjunctions are going to happen in 2012.

The Moon

The principal phases of the moon this month are:

Full moon

Last quarter

New moon

First quarter

 July 5th  July 11th  July 19th  July 26th
The Planets

Mercury is an evening sky object and it reaches greatest elongation east of the Sun on July 1st at 26°E.  However, it sets shortly after sunset so is not likely to be seen by UK astronomers.

Venus is a brilliant object, at magnitude -4.5, in the morning sky among the stars of Taurus.  During the first week of July it lies not far from Jupiter and the Hyades star cluster.  Telescopes show Venus to be a waxing crescent and the angular diameter is decreasing as the planet recedes from Earth.

Looking northeast on July 7th at 3.10am. Aldebaran, Venus, Jupiter and the Pleiades make an almost perfect line in the sky.

Earth is at aphelion – the furthest distance from the Sun of the year – on July 5th.  The distance between our planet and the Sun will be just over 94½ million miles that day.

Mars is an evening sky object in the constellation Virgo.  The Red Planet is a fading first magnitude star and it is visible low in the western sky for a few hours after twilight has fallen.  Mars is now so distant that high magnifications on well collimated telescopes will be required to see the gibbous disk of the planet.

Jupiter is shining at magnitude -2.1 in the morning sky before dawn.  It will rise a little earlier each successive morning.  Look for Jupiter near the northeast horizon near the more brilliant Venus.  On the morning of July 15th the Moon will pass very close to Jupiter.  From the south of England the moon will graze [PDF] or completely cover the distant planet.  From Northumberland we’ll get the kind of view shown below.

Crescent moon and Jupiter at 3.10am on July 15th as seen from Northumberland.

Saturn is visible in the evening sky, in the west, for a few hours after dark.  Saturn is marginally brighter than Mars, also in the constellation Virgo.  Saturn is to be found 5 degrees north of Spica, the brightest star in Virgo and about the same magnitude as Saturn.

Uranus and Neptune are best seen in the morning sky.  Uranus shines like a star of magnitude +5.8 in the constellation Pisces.  From exceptionally dark skies Uranus is a naked eye object for keen sighted observers.  Binoculars will show it easily enough.  Neptune resides in the constellation Aquarius and telescopes are required to see this 8th magnitude planet.  The positions of both worlds are shown in the chart below:

Positions of Uranus and Neptune at 2am on July 15th 2012.

The thin sliver of a crescent moon looked marvellous in the evening sky.  Here’s a picture I snapped with the Nikon D80 from the back garden earlier:

Venus was nearby but a little lower in the sky and hidden by those nearby houses.

The appearance of the ‘fingernail’ moon also means that on each successive evening the moon will be above the horizon a little longer and as the phase increases, the sky will be filled with more and more moonlight.  This will make the forthcoming Geminid meteor shower difficult to observe when it peaks next month.  On the upside, the full moon a couple of days earlier will culminate in a total lunar eclipse on the 10th.  More about that soon.