Rho Aquilae: A Not-So-Proper Motion

There’s a famous building in Exeter, presently located off of Fore Street. It was built some time in the 15th or 16th Centuries, but found itself under threat in the early 1960s from a new road development. In 1961, rather then have this listed building demolished, the Council paid to have it moved. And move it did; the entire house was put on a timber cradle and shifted 70 metres to the bottom of West Street. And there it remains to this day. (There’s a video of the move here.)

One of the occupational perils of astronomy is people wittering about their star signs. Unfortunately, to an astronomer, constellations are next to useless. You see, people tend to assume that stars are like houses – they don’t move. But, as we just saw with Number 16 Edmund Street above, every now and then houses can do exactly that, so the common sense approach is not the whole story.

In fact, unlike most houses, stars are moving all the time.

It’s just that this change seems small from the immense distances we see them at. As such, most of these ‘proper motions’ are not visible to the eye over the course of a human lifespan. In fact, most of the constellations haven’t changed much over the course of recorded human history. The zodiacal constellations come to us from the Ancient Greeks and thus haven’t changed much in at least two to three thousand years (with the exception of the odd deletion or border change).

However, over time, the constellations do drift. This is why they’re no use to professional astronomy; they’re at best no more than line-of-sight effects. Come back in 200,000 or 2,000,000 years and Orion and Ursa Major will be long gone. But of course, very occasionally, notable changes can happen rather faster.

This brings us to Rho Aquilae.

Rho Aquilae is a main sequence star of type A2, which means that it’s pretty similar to Sirius. It’s brighter, hotter and whiter than the Sun. It’s also younger; stars of this type don’t live for more then about a couple of billion years. It’s located 154 light-years away. When it was named centuries ago, it was comfortably inside the borders of Aquila, the Eagle. Unfortunately for the Eagle, however, Rho Aquilae quietly carried on moving. (Bad star – no biscuit!) In 1999, just in time for the Millennium, it crossed over into Delphinus.

So in fact the Eagle’s star has been stolen by the Dolphin. Bad Dolphin. Of course, re-naming Rho Aquilae would just confuse things even more, so it’s kept its old name. However, it does demonstrate the practical limitations of the concept of constellations!


2 Responses to “Rho Aquilae: A Not-So-Proper Motion”

  1. Sarah Waldock Says:

    I may be a Brit but I follow American football and I’m afraid I was having visions of the Miami Dolphins kidnapping the quarterback of the Philadelphia Eagles as an analogy [bad Sarah, definitely no biscuit]…… but what a nice proof of the futility of Astrology.
    I always claim that my star sign [if asked] is Derry and Toms [Monty Python reference].

    • LOL, that’s a good answer!

      I tell people I’m a physicist rather than an astronomer, so they don’t ask me to tell them their horoscope! (Or their UFO story … which is so often ‘Venus or a plane, plus alcohol’!)

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