During the first week of September, observers have an opportunity to see a bright near-Earth asteroid known as 3122 Florence (aka 1981 ET3) as it sails by our planet. Some 4.4 kilometres (2.7 miles) in diameter and orbiting the Sun every 2⅓ years, this Amor-group asteroid was discovered in 1981 and is the fourth-largest NEO known.
Around midday UK time on 1 September, 3122 Florence passed little more than 7 million kilometres (4.39 million miles), or 18 lunar distances, from Earth. This was its closest approach since 1890 and the closest it will get for 500 years. Given its size and current proximity, the asteroid will be brighter than the tenth magnitude for the next few days as it tracks across the Milky Way, hence it’s easy object for small telescopes despite the light of a waxing gibbous Moon.
On the UK night of 2—3 September (Saturday night, Sunday morning) 3122 Florence lies 0.049 astronomical units (7.33 million kilometres) from our planet, travelling at a rate of 9 degrees/day against the stars of Delphinus, a small constellation better known as the dolphin. The following wide-angle star chart shows you how to locate Delphinus.Once you have located Delphinus with the naked eye in relation to the Summer Triangle of stars Deneb, Vega and Altair (avoiding streetlights and masking the 11-day-old waxing gibbous Moon with a tree or wall will help you locate the principal stars outlining the dolphin), turn your telescope towards the diamond-shaped four-star “Job’s Coffin” asterism defining the head of Delphinus, starting with your lowest power eyepiece.
On the night in question, 3122 Florence is predicted to shine at magnitude +9.1, considerably brighter than the faintest field stars shown in the detailed finder chart below. For scale, this chart is about five degrees wide — roughly the field of view of a 10×50 binocular. The asteroid is moving at a rate of about 9 degrees/day this night, or slightly more than 22 arcseconds/minute. Put simply, it’s travelling at a rate equal to the apparent angular size of planet Jupiter every two minutes. In 10cm aperture telescopes and larger at magnifications of 100x or more, the realtime motion of 3122 Florence should be readily observable.