Where to Go

Right, so now you’ve chosen your weapon, where are you going to stick it? Cue the inevitable Uranus jokes.

One of the most important things to decide is where to actually do your astronomy. You don’t really want to stray too far from home, but you do at least want to make the trip outside worthwhile. Balancing these two factors is a tricky thing.

First, let’s consider what dark means. You’ll probably think of it as an absence of light, but your astronomer would think of it in terms of how many things in space they can see. The darker a place is the better it is for doing astronomy. We can’t always get to somewhere properly dark but now is a good time to find out where the darkest place near you is. There’s also some jargon we need to get used to:

• Magnitude  – how bright an object is. The lower the magnitude, the brighter the object. Just to confuse matters further it’s on a logarithmic scale, so a magnitude difference of one represents a roughly 2.5x difference in brightness. Confused? Don’t be. This blog deals in terms of what you can see with a street light next to you etc. I won’t be using numbers so you don’t need to breathe into the paper bag any more.
• Naked Eye Limiting Magnitude (NELM) – the magnitude of the dimmest thing you can see. NELM in the city is terrible – in central London you’ll be lucky to see more than about 15 stars on a clear night. In Bushy Park I can pick out all the constellations, so the NELM is lower (i.e. I can see dimmer objects).
• Light Pollution (LP) – the bane of our astronomical experience. Streetlights are there to light the ground but half their light goes into the sky. The sodium in the lamps has that awful orange hue, and you can buy filters for telescopes to combat it.

The horrors of Light Pollution - seen here from the doorstep obscuring Ursa Minor.

 Transparency and Seeing – not all nights are the same. The differences in the quality of sky are down to amount of cloud (Transparency) and atmospheric turbulence (Seeing).

• Dark Adaption – the rod cells in the retina are extremely sensitive, especially to blue light. These are the cells that allow you to see in the dark because the colour sensitive cones need lots of light to work. When the rods are exposed to brighter light sources then they get overloaded and stop working. It takes roughly half an hour for the neurotransmitting chemicals in them to settle down, so the longer you stay in the dark, the better you can see. This is also why astronomers use a red light, to avoid bleaching these cells.
• Surface brightness – Very important for urban astronomy, this is a measure of how spread out the light is coming from the object. As an example, there are two galaxies I looked for one night in the park. M33 is bright, around mag. 5, while M77 is dim at mag. 8. But I could see M77 and not M33 – the reason is that M33 is very much bigger so the light is so spread out you can’t pick out the galaxy against the light pollution. M33 is notorious for being hard to spot – go to the dark of somewhere like Exmoor and it’s very easy to see, but even a little light pollution will make it impossible. This is also where dark adaption is important.

So the take home point is that the less light the better, and that also the choice of objects is key. I’m only going to highlight things that can be seen with NELM of about 4.0 – 4.5, a typical suburban sky. If you’re somewhere rural your views will be very much better.

So where do I do most of my star-gazing?

·        The front step – our flat is blessed with a small patch of gravel, just large enough for the ST80. There is a street light right next to it, so I don’t need a torch to look at the star map. It benefits from being close to the flat but the light pollution is pretty bad and I have severely restricted view of the sky. Bushy Park – Away from the street lighting, the park does get pretty dark. There are a few other considerations if you’re going somewhere like this:

o   It’s a way from the flat so I am limited as to what I can lug down there.

o   They close it after 10.30pm in September and November to cull the deer.

o   It’s full of deer – being native to the New Forest I’m used to deer. They don’t see me as a threat so I can happily observe with them right next to me.

o   A bigger concern is violent humans. Everyone I’ve met while out star-gazing has been either friendly or done that London thing of ignoring the strange chap in case he starts trying to converse… Be sensible – always tell someone where you’re going and if possible go with someone else. If you’re not sure then don’t go – you might not pick out the Crab Nebula on your front porch but it’s far more important to feel safe.

Bushy is the darkest place in Greater London I know of – On one extremely transparent night I did manage to pick out the Milky Way as a faint glow overhead. This is a very rare occurrence, but just goes to show that it's always worth a try!