David Vincent (DVO’s Resident Astronomer and Puzzlemeister) sets the record straight!
My local running group know that I can be a bit of a bore when we meet on clear nights. They listen good-naturedly while I point out various stars and planets. A recent question was “Aren’t all the planets aligned at the moment?” Indeed, the media have picked up this line recently: “See the rare planetary alignment!” – etc. It’s rather irksome. The planets are always “aligned”, and always have been. We live on the flat disc of the Solar System. All the planets orbit the Sun pretty much on the same plane – after all, they all came into being at approximately the same time due to the same process – the gravitational collapse of a spinning mass of gas and dust. All the planets orbit and spin in the same direction (research the exceptions to this if you’ve got the time). From our position within this plane, the other planets, the Moon, and the Sun, will all be somewhere along a line – called the ecliptic – the plane of our orbit, and recognised from ancient times with the zodiacal band – the star groups through which the Moon and planets pass.
Another frequent question is: “How do you know that that is a planet?”. Two hundred years ago, before street lighting and easy entertainment, this would have been a strange question indeed. There are only five planets visible in the night sky, and we can reduce that to four if we discount Mercury – only ever visible as a bright twinkle very low, close to the horizon, in a bright sky just after sunset or before sunrise. Those four – Venus, Mars, Jupiter, and Saturn – are easy to distinguish from each other and the stars around them with a little practice. The first three are all brighter than any stars in our night sky, and have quite a distinctive colour – Venus: amazingly bright (said to be the only celestial object apart from the Sun and Moon capable of casting a shadow); Mars: bright and distinctly reddish; Jupiter: very bright, and yellowish. Saturn could be mistaken for a bright star except that – well, read on…
The stars and the planets appear to us as discs, of course. Without a telescope, they appear as mere point sources of light, because those discs are tiny from our distance. The angle made at your eye when looking at the two sides of Jupiter, for example, is around 50 seconds of arc (an arcsecond is one 3,600th of a degree). The smallest angle that a human with good eyesight can discern is about one minute of arc – a sixtieth of a degree, or sixty arcseconds. So, we can almost see Jupiter as a disc. The star with the largest angular diameter however, makes an angle measured in only milli-arcseconds. One of the largest in apparent size is Betelgeuse, which many of you will know as the bright shoulder star of Orion, with an angular diameter of about 50 mas.
This means that the stars are effectively point sources of light. One can imagine a single line of photons streaming from a star to your eye. On entering the atmosphere, those photons are subject to absorption and refraction. The star therefore appears to appear and disappear rapidly as its photons are scattered out of our line of sight, before the stream is restored once again. In other words – the stars twinkle.
The much larger angular size of the planets mean that we receive multiple streams of photons. If one is tweaked out of our line of sight by the atmosphere, we still receive the others. The planets do not twinkle – they shine with a steady light!
Now pop outside and find Mars and Jupiter – high in the south-ish – and see if you can tell the difference. If you’ve read this far (well done), you might enjoy this XKCD cartoon on the subject.
Fun fact the Editor heard on Radio 4: John Flamsteed (Astronomer Royal) was expelled from the Royal Society, ostensibly for not paying his membership fee. But apparently he didn’t get on with Sir Isaac Newton, so that was maybe just an excuse!
The context for this item was the fact that Elon Musk was elected as a Fellow of the Royal Society in 2018 for his advances in space travel and electric vehicles, but recently thousands of scientists have expressed concerns over his “public pronouncements and behaviours” (Reuters).
John Flamsteed’s Derby house at 27 Queen Street has been occupied by squatters for decades, but has this year been granted a Blue Plaque and a Grade II listing. It was build in 1669 by his father Stephen Flamsteed, who lived there with his death in 1675, the year John was appointed Astronomer Royal. A campaign group Friends of No 27 Queen Street, Derby is fundraising to turn the house into a Museum, as there have been other notable occupants over the years, including the clock-maker John Smith (the blue structure in the roof arch is actually a double-faced clock).
Fun fact No 2: The Greenwich Meridian – and the coordinate system in use today – was invented by John Flamsteed to take accurate measurements of the stars, and was originally the Meridiem Derbensis. If Flamsteed hadn’t moved to London in 1775, we might, here in Derbyshire, be sitting on the Prime Meridian.
