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Fri, 01 Sep 2017

Tracking Florence

Trail of asteroid 3122 Florence passing Earth. Credit Robin Scagell

Trail of asteroid 3122 Florence on 2 September at 23.25 BST. A composite of 20 15-second exposures taken with Canon 70D camera through 80 mm refractor, 600 mm focal length. Photo: Robin Scagell

A near-Earth asteroid, 3122 Florence, is currently skimming past Earth and will be visible in small telescopes and possibly large binoculars for the next few days. 

The asteroid, named after Florence Nightingale, has been known about since its discovery in 1981, and while its orbit brings it comparatively close to that of Earth, this is the nearest it will be to our planet for several centuries. Being only about 5 km across, there is no risk of any interaction between the asteroid and Earth, apart from the flurry of activity among amateur astronomers who want to observe it.

If you glance out on the next few evenings and spot something moving across the sky, it isn't Florence but a plane, a satellite, a shooting star or a UFO (that's a joke!). No, you'll need to be fairly clued up about finding objects in the sky, have a suitable telescope or decent binoculars and able to recognise the constellations. The gibbous Moon will also be in the sky in the earlier part of the evening, making things a bit more difficult by hiding the fainter stars. But for those who want to have a go, here are some charts to help you.

The first shows the general view of the mid sky looking south at about 10 pm BST. Sky-watchers will recognise the three bright stars Vega, Deneb and Altair, known as the Summer Triangle. The track of the asteroid is shown with positions marked for 0h (midnight) BST at the start of the day in question as the object moves from south to north. The three areas marked are covered by detailed maps which you can access through the links below the map, which show the track of the object at hourly intervals. These show stars to magnitude 10. Use them to pinpoint the area of sky to view with your telescope. The maps are all produced using SkyMap.

Photographers should use long focal lengths and time exposures, ideally tracking the stars using a driven telescope mount. Use an ISO setting around ISO 400, and give exposure times of around 30 seconds or longer to capture the trail of Florence. Alternatively, use shorter exposures and combine the results using image-stacking software. Auto-focus lenses should first be focused on a bright star such as Vega, then switched to manual, or you can use live view to focus on a bright star while on manual. 

Detailed map for 2 September

Detailed map for 3 September

Detailed map for 4-5 September

And, for the really dedicated, as the asteroid will be quite faint:

Detailed map for 6-7 September


Added by: Robin Scagell