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Mon, 16 Oct 2017


Keep an eye on Iris


Photo of asteroid Iris
Iris, photographed on 11 October using a telephoto lens
If you want to se an asteroid, and are fed up with non-existent meteor showers and impossibly faint comets, try looking for asteroid Iris. It’s easily found at the moment, and you can see it with binoculars.

You might not have heard of Iris, but it is actually the fourth brightest asteroid. It was the seventh ever to be discovered, after Vesta, Ceres and Pallas. Its discoverer was J R Hind, in 1847, from a private observatory in Regent’s Park, London.

This is a particularly good time to look for Iris, as at the end of this month it reaches its maximum possible brightness of 6.9. That is below naked-eye visibility, but easily reachable with even small binoculars. But you have to know exactly where to look.

Fortunately it is passing through the constellation of Aries, which is currently well placed looking quite high in the sky in the south-east during the late evening as seen from the UK. While not a bright constellation, it’s easily found by anyone with a basic knowledge of the sky. Its three brightest stars make an easily recognisable group. Use the map below first to find Aries itself.  With binoculars, first identify the three main stars of Aries, then use our map to ‘star-hop’ through the patterns of fainter stars to Iris.

Although Iris is about 130 million miles away, it moves noticeably against the starry background from night to night, so if you want to pick it out you need to know its position when you are looking. Bear in mind that the tick marks on the map are for midnight at the start of each day, so if you are observing before midnight, the asteroid will be close to the next day’s mark. 
Finder chart for Aries
Use the well known star cluster of the Pleiades, in the east, to locate Aries
Positions for asteroid Iris during October 2017
Positions for Iris as it moves through Aries during October 2017
Photographing Iris
Just a few seconds’ exposure on a decent camera will show Iris, but you’ll need to support your camera on a firm tripod and make sure the lens is focused on infinity. Some cameras with auto-focus may not be able to focus as the stars are too faint, so you may need to focus first on a bright star using auto-focus and then switch off the auto-focus.

With exposures longer than a few seconds, and telephoto lens settings, the stars will start to trail after more than about four seconds’ exposure as the sky moves (or, more accurately, the Earth turns). So to keep the images as points of light, use a short exposure time and a high ISO setting.

The photograph at top was made using a 125 mm lens setting on a Canon 70D camera, with a 75-second exposure. The camera was driven to follow the stars. The image is cropped.
 

Added by: Robin Scagell