Crescent Moon and Venus in the Twilight

Following on from my last post, I’ve been able to spot Venus after sunset on several occasions over the past week thanks to the clear skies we’ve been enjoying. Tonight, the twilight sky was enhanced by the arrival of a young crescent Moon, about the same height above the horizon as Venus but about a hand-span to the right. We had quite a bit of cloud and some rain during the afternoon, so I wasn’t sure if I’d be able to see the two objects together; however the sky began to clear around sunset and as it got dark there was a nice strip of cloud-free sky  in just the right place.

Since Venus and the Moon were so far apart, I could only fit them both in by taking quite a wide angle shot, so the Moon looks pretty small in the photo. It looked so lovely, though, that I felt it deserved a close-up picture too. The image below shows the Moon above the Rail Bridge, with the yellow light of the low Moon reflected in the River Tay – I’m rather pleased with this one!

Tomorrow night – Wednesday 6th – the Moon will have moved higher in the sky and will appear to the upper right of Venus, and a bit closer. If the sky is clear I may try to take a comparison shot and post it too!

NB – I’ve also finally got round to updating my Night Sky notes for November – just click on ‘The Sky This Month’ tab at the top of the page to find out what to look out for over the coming weeks.

The Not-Quite-SuperMoon

It would appear that the ‘Super Moon’ craze is with us again, despite it actually being a ‘so-marginally-larger-that-you-won’t-really-notice-the-difference Moon’. Yes, tonight/tomorrow night’s Full Moon (the actual time of Full Moon is 1132 GMT on 23 June, so it will look pretty much equally full tonight and tomorrow night)  is the closest of the year, but it is only very slightly larger than the average Full Moon. For example, this Full Moon is only 0.6% larger than the one in May, and equally only 0.6% larger than next month’s in July; and this difference is not enough to be noticeable to the naked eye.

The Moon does indeed vary in distance from the Earth as it orbits us once a month, from a distance of 356,700 km (221,600 miles) at perigee (closest to Earth) and 406,300 km (252,500 miles) at apogee (furthest from Earth). This cycle lasts just over 27½ days , a bit less than the time it takes for the Moon to go through a complete cycle of phases, which is 29½ days. As a result, the point at which the Moon is closest to us each month doesn’t coincide with a particular phase – only about once a year will perigee (or apogee) line up with a specific phase. So usually there is one Full Moon a year which falls near the Moon’s closest point to the Earth, and one which falls nearest the most distant. In 2013, the Full Moon of June is closest, and the Full Moon of  December the most distant.

However, the size of the Full Moon varies gradually from largest to smallest over several months, so the average Full Moon lies between these extremes. To suggest that the Full Moon of June is going to appear strikingly larger than the usual Full Moon is a gross exaggeration – in fact, it is just less than 7% larger than the average, which is at the limit of the size difference noticeable to the eye.

Something that might make the Full Moon of June appear larger is the well-known ‘Moon Illusion‘, whereby the Full Moon looks bigger when it is seen low in the sky, as it will be this month. This effect has been observed since ancient times and is purely an optical illusion, probably resulting from it being seen alongside foreground object near the horizon.

For a more detailed discussion of the ‘Super Moon’ myth, there’s a nice article on the Sky and Telescope website.

Remembering Neil Armstrong

I didn’t write anything at the time of Neil Armstrong’s death a few weeks ago, so as a belated tribute, here’s a touching collection of memories from his NASA colleagues, including fellow astronauts.

Once in a Blue Moon

Today’s Full Moon is the second this month – the Moon was also full on Thursday August 2nd. In recent years, the second Full Moon in a single calendar month has become known as a ‘Blue Moon’, as in something that happens only rarely. However, this occurrence is not especially rare – we get two Full Moons in a calendar month every two or three years – and also the definition of an astronomical ‘Blue Moon’ is slightly more complex than this straightforward definition.

It’s possible to get two Full Moons (or two of any other particular phase) in a month because the period between two similar phases – New Moon to New Moon, say, or Last Quarter to Last Quarter – is just a little over 29½ days. This is known as the Moon’s synodic period, and as it is slightly shorter than most calendar months, a phase that occurs at the beginning of the month can be repeated again at the end. February is the only month that is too short for this effect – in fact, since February’s 28-day length is less than the Moon’s synodic period, it’s possible for February to miss out altogether on a particular phase. This was notable in 1999, when both January and March had two Full Moons and February had none (if you missed that interesting event, it will happen again in 2018). Even this year, February lost out on a First Quarter, with one falling on January 31st and the next on March 1st.

So where did the ‘Blue Moon’ description come from? Well, it all started with Sky and Telescope magazine, which ran an article in its March 1946 issue in which the author referred to a ‘Blue Moon’ as the second Full Moon in a month. This then became accepted lore in future issues of the magazine. However, the author of the 1946 article had misinterpreted a definition given in the American Farmer’s Almanac, which appears to be the original source of the expression. It’s perhaps not surprising that this happened – the definition given by the Farmer’s Almanac is rather convoluted.

Basically, the original definition says that in years when there are 13 Full Moons rather than the usual 12, and one of the seasons has four Full Moons while the others have just three, the ‘Blue Moon’ is the third one in the season with the ‘extra’ Full Moon. The full story can be found in this Sky and Telescope article, which explains it all in more detail than I can here.

Nonetheless, even though this may be the ‘correct’ definition of a ‘Blue Moon’, I think it’s too complicated to catch on and the ‘second Full Moon in a calendar month’ version will be the one that’s most often used. So watch out for August’s ‘Blue Moon’ – the next will be in July 2015.

Old Moon in the New Moon’s Arms

There was a lovely crescent Moon shining low in the southwest  at sunset tonight, the first chance I’ve had to see the Moon this lunation thanks to the poor weather over the past few days. New Moon – when the Moon is over in the same direction as the Sun – was on Saturday 6th, but at this time of year as the young Moon moves away from the Sun its path lies low above the horizon, so it usually takes a few days before it becomes visible in the evening sky. Even so, I would have spotted it last night if only it had been clear!

One of the striking features of a crescent Moon is the effect popularly known as ‘the Old Moon in the New Moon’s Arms’, when the nighttime side of the Moon can be seen shining faintly beside the bright crescent. Technically, this is called Earthshine – while the brightly-lit crescent is receiving its light directly from the Sun,  the illumination of the rest of the disc comes from light reflected from the Earth (second-hand sunlight, if you like). When the Moon is at a crescent phase in our sky, the Earth appears almost full in the Moon’s sky; as well as being four times the diameter that the Full Moon looks to us, the Earth’s generally cloudy atmosphere is very reflective, so there’s lots of sunlight bouncing back in the direction of the Moon. As a result, Earthshine is often bright enough to show clearly the main features on the lunar disc.

Earthshine gradually becomes less obvious as the crescent Moon gets wider. Partly this is because it gets drowned out by the brighter sunlit side, but also because the Earth is waning in the Moon’s sky so there’s less light being reflected Moonward. However, if you block out the sunlit part of the Moon with your hand or the side of a building you can still see the Earthshine right up to at least First Quarter – in fact, using binoculars I’ve detected its faint glow a day or two after half Moon.

The same effect can be seen around three weeks later when the waning crescent Moon appears in the dawn sky shortly before New Moon – I suppose it should then be called ‘the New Moon in the Old Moon’s Arms’?

Hunter’s Moon

It’s Full Moon today, with the Moon shining among the faint stars of the zodiacal constellation of Aries. As this is the first Full Moon after the Harvest Moon – the one closest to the autumn equinox, which fell on September 23rd – it’s often known as the Hunter’s Moon.  The idea was that the full-moonlight helped hunters find and chase foxes across the newly-harvested fields, even after sunset.

As we move through autumn into winter, each consecutive Full Moon is rising further to the north of east, and is higher in the south at midnight; in fact, the Full Moons are doing the exact opposite of the Sun, which is now getting lower in the sky, and rising and setting further to the south of due east and west. This will continue until we reach the winter solstice on December 21st.

 Over the next few nights the Moon will travel further east, and on Monday 25th will lie around 5 degrees – about half a hand’s-span – to the lower left of the Pleiades star cluster, and about the same distance to the upper right of Aldebaran, the bright red giant star that  marks the eye of Taurus the Bull. By the then the Moon will be noticeably waning and will again present some nicely shadow-filled craters when viewed with binoculars or a telescope.