The Sky this Month

I’ve added a new page to the blog, The Sky This Month, which you can view using the link on the right. I’ll update this every month to keep you informed of the main objects of interest in the night sky. I’m also aiming to get back to posting on the main blog page more regularly!

Counting Stars in Orion

The Campaign for Dark Skies (CfDS) and Council to Protect Rural England (CPRE) are running a star count over the next week to determine the level of light pollution across the UK. This involves counting the number of stars you can see in the constellation of Orion, within the rectangle bounded by the four bright stars Betelgeuse, Bellatrix (gamma Orionis), Rigel and Saiph (kappa Orionis).

As it was a nice clear, cloud-free evening I decided to go out into the back garden tonight and try it for myself. I live in a dark(ish) street in a well-lit suburb that usually gives me dark enough skies to happily observe from home. On a crisp, clear moon-free night I can normally see the Milky Way and some of the brighter deep-sky objects, such as the Andromeda Galaxy and the Double Cluster in Perseus, with the naked eye.

So how did I fare tonight? Well, it was slightly hazy and the Milky Way could only be glimpsed overhead where it passes through Perseus (and I just got a suggestion of the Double Cluster with averted vision). I gave myself 15 minutes to dark adapt and then counted the stars I could see in the designated area within Orion…and it came to a grand total of 10. I’ve no idea how this compares to other parts of the country, so it will be interesting to see the final results of this experiment. But I do know that I’ll be counting stars in Orion every time I find myself under a clear moonless sky at other locations, just for my own curiosity!

If you want to take part in the survey yourself, you can find out more (and submit your results) at the BAA/CfDS Orion Starcount webpage.

Morning Star

The weather has been pretty poor this week, with overcast skies and rain. The only exception was Monday, which started bright and frosty and remained clear and sunny all day. I got my first view of Venus in the dawn sky, shining brilliantly about 12 degrees up in the south east. Venus has moved rapidly into the morning sky after passing through inferior conjunction on October 29th - it was only a few degrees above the horizon in early November, but by the end of the month will be rising over 3 hours ahead of the Sun, when we’ll be able to see it blazing down from a dark pre-dawn sky.

The period around inferior conjunction is when Venus is at its closest to Earth, so appears largest. Since it’s also close to the Sun in the sky, the planet is illuminated from one side and shows a narrow crescent that can be easily seen in a telescope and even a pair of binoculars – I had no difficulty seeing the crescent shape of Venus on Monday morning using my 10×50 binoculars.

This is also a good time to see if you can spot Venus in full daylight – just follow the planet as the sky brightens and you should still be able to see it after sunrise. By noting its position in relation to treetops and rooftops, I could still find Venus 30 minutes after the Sun came up. Try it!

Encounters with Saturn

Exactly 30 years ago today, on the 12th November 1980, the Voyager 1 spacecraft flew past Saturn and returned a stunning array of images of the giant planet, its rings and moons. At its closest approach, Voyager 1 passed just 77,000 miles (124,000 kilometres) above Saturn’s cloud tops. The images that the spacecraft sent back to Earth – around 20,000 in total – revealed detail never before seen.

Although not as colourful or complex as the clouds that Voyager recorded at Jupiter 20 months earlier, the belts and spots it saw in Saturn’s atmosphere far surpassed the view through the best ground-based telescopes. Early measurements showed Saturn to be one of the windiest places in the Solar System – equatorial wind speeds reached over 1000 miles per hour, four times the wind speeds in Jupiter’s equatorial zone – while the ring system was revealed to be made up of hundreds of individual ‘ringlets’, even appearing in the regions that looked like empty gaps when observed from Earth.

Some of the most exciting images came from the flybys of Saturn’s larger moons, although the biggest moon, Titan, was something of a disappointment  as its surface was frustratingly shrouded in cloud. Dione and Rhea showed cratering, but some of the impacts looked like they’d been partly eradicated by material flowing out from the moons’ interiors. The greatest surprise from Voyager 1 was arguably the remarkable images of its moon Mimas, just 240 miles (390 km) across, but with a giant impact crater 80 miles (130 km) wide on its surface, presenting a view reminiscent of either a giant eye, or, for sci-fi buffs, the Death Star from Star Wars.

As it left Saturn, Voyager 1 took the image shown at the top of this post – a crescent planet casting its shadow across the rings, a view impossible to see from Earth. In order to make its close fly-by of Titan, the spacecraft was placed into a trajectory that would take it above the orbital plane of the remaining planets in the solar system and send it out into interstellar space.  Today, 30 years later, Voyager 1 is 10.7 billion miles (17.2 billion km) from the Sun, and travelling at almost 10 miles per second (15 km/s). Its next encounter will be with the star AC+79 3888 in the constellation of Camelopardalis in about 40,000 years’ time – although its fly-by distance on that occasion will be a considerable 1.6 light years.

If you don’t mind getting up early, you can celebrate the Voyager 1 anniversary by observing Saturn for yourself. The distant planet looks like brightish yellow ‘star’ in the pre-dawn sky, rising east just before 4 am and standing about a hand-span above the south-eastern horizon as the sky begins to brighten an hour before sunrise. Don’t confuse Saturn with the planet Venus – it’s also in the south-east in the morning twilight, but is much lower and much brighter!

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.

 

Welcome to Keep Watching the Skies!

In this blog I’ll be writing about all kinds of sky-related matters – things to look for in the daytime or nighttime sky, topics of general astronomical or meteorological interest, and of course the science-fictional aspects too. The title of the blog covers it all, and, if you don’t know, is taken from Howard Hawks’ classic 1951 sci-fi film The Thing from Another World. At the end of the film, with The Thing finally defeated, newspaper reporter Scotty is at last able to broadcast the story of humanity’s first encounter with an alien visitor to the waiting world. He ends his report with a warning…”every one of you listening to my voice, tell the world. Tell this to everybody wherever they are. Watch the skies, everywhere. Keep looking. Keep watching the skies!”

Good advice, which I hope you’ll follow!