Astroblog: What is this thing called ZHR?

Wednesday, August 13, 2008

What is this thing called ZHR?

I didn't see any Perseids, here is the reason why. Mist. Not thick Orgulous mist but mist just the same. The tree up the end is the tree that features in a lot of my shots.

Not that I would have seen much anyway, although the Perseids are a good shower, with a ZHR of 100, from where I am there is not much Z in the ZHR.

What is the ZHR anyway. ZHR stands for Zenithal Hourly Rate. This is the theoretical number of meteors you could see if you have normal eyesight, and are observing in a dark location away from city lights, the area you are observing is complete free of cloud or other obstacles, and if the meteor shower is occurring directly overhead.

Now my eyesight isn't too bad, I'm not too far from the city, but these nights, when the sky isn't covered by cloud or fog, the sky is achingly clear and you can see the Milky Way with its dust lanes clearly. Not as spectacular as in the country, but still pretty decent.

The killer is the Zenith bit. ZHR's are calculated as if the meteor showers were happening overhead. Dead overhead, the sky is more transparent, as you look towards the horizon you have too look through more atmosphere, and there is more dust and junk in the atmosphere lower down as well, so you will miss the dimmer meteors. Also, closer to the horizon, there is more of a chance you will miss meteors that will be start their burn behind things cluttering up the horizon (like hills, trees, neighbour's rooves etc) .

For example, if the radiant (the apparent spot in the sky where the meteors can be traced back to) is 30 degrees above the horizon (that's about 5 hand spans above the horizon), then you will see half as many meteors than if the radiant was at the zenith.

Now where I am, the Perseid radiant is just below the horizon, so automatically I'll lose about half the meteors, as they will be below my horizon. And so low to the horizon, all but the brightest will be lost in the murk, so I'd probably see around 1/10 the meteors you would see at the zenith. With the Perseids expected to be around a ZHR of 100, the best I could expect to see would be around 5 meteors an hour, shooting up from the horizon (even then that is an exaggeration, as I have left out other factors). People to the north of me will see more, but only in places as far north as Darwin will see something like the European or US observers will.

Still, one or two meteors would be a nice start to the day. It does look like the Perseids are on track for a 100 ZHR, and here's an image from someone who saw a lot of meteors during the night.

Labels: Meteors