Under optimal conditions, how many planets can you see with the unaided eye? Aside from the one beneath your feet, most observers would say five: Mercury, Venus, Mars, Jupiter and Saturn. If we assume a naked-eye limit of magnitude +6 for a keen sighted individual from a location devoid of moonlight and light-pollution, there is one other planet we can add to the list – Uranus*.
Although Uranus holds the distinction of the first planetary discovery, by Sir William Herschel on 13 March 1781, its relative brightness means that it was observed by other astronomers on a number of previous occasions, but mistaken for a star owing to its slow orbital motion. The planet was first definitely seen by Astronomer Royal John Flamsteed in December 1690. In fact, he saw it on at least six occasions and catalogued it as a star — 34 Tauri.
In fairness to these eminent astronomers of yesteryear, if Uranus is observed at one of its stationary points between retrograde and prograde motion then it could easily be mistaken for a faint naked-eye star. However, Somewhat embarrassingly for the French astronomer Pierre Lemonnier, records reveal that he saw Uranus at least twelve times between 1750 and 1769 without realising its planetary nature!
With a diameter four times that of the Earth, Uranus is the third-largest planet in the Solar System. It orbits the Sun once every 84 years at a distance averaging 2.875 billion kilometres, or 19 times farther away from the Sun than Earth. In common with its more distant sibling, Neptune, Uranus is considered to be an ‘ice giant’ because, in addition to being largely composed of hydrogen and helium gas like its larger kin Jupiter and Saturn, Uranus also contains methane gas plus water and ammonia in the form of ice.
With a minimum temperature of −224.2 °C, Uranus’ atmosphere is the coldest in the Solar System, yet still experiences wind speeds in excess of 800 kilometres per hour. A Uranian day is also 17¼ hours long. Furthermore, the planet’s axis of rotation is tipped to one side, almost in the plane of its orbit. With its polar regions positioned where most planets have their equators, the seasons of Uranus are odd indeed. The planet has a retinue of 27 known moons and a system of rings that were discovered in March 1977.
On the morning of Monday, 28 October, Uranus comes to opposition in the constellation of Aries. The seventh planet from the Sun is closest to Earth this year at 20:37 UT on 27 October, when the centres of our two worlds will be 18.833 astronomical units or 2,817 million kilometres (1,751 million miles) apart.
Observing Uranus with the naked eye or binoculars
We’ve learned that Uranus was historically mistaken for a star several times, but can most people see it today with the naked eye if they know exactly where to look? I think everyone should have a go. Using the PDF chart from the top of the page (or click here), find a dark location away from streetlights on a moonless night and let your eyes get dark-adapted over the course of at least 15 minutes before making the attempt.
As viewed from rural Norfolk under better than average conditions on the night of 27 October, I found that Uranus was not difficult to see with averted vision. For me, glancing at ξ1 Ceti (magnitude +4.4) but concentrating on Uranus’ position would occasionally make the planet pop into view. Tip: if you are slightly long-sighted (presbyopic) and wear glasses for everyday use as I do, try removing them at night — you may find that ‘night myopia’ restores your 20/20 vision for the sharpest views of the stars. Don’t worry if you can’t see Uranus without optical aid, because even the smallest binocular will reveal it.
Observing Uranus with a telescope
If Uranus seems star-like in binoculars, then the view doesn’t improve dramatically in a telescope. However, in medium-sized amateur instruments, the cyan colour of Uranus’ tiny disc can be seen. Despite possessing a diameter four times that of the Earth, its great distance means that its disc appears just 3.6 arcseconds across. So small, in fact, that you would need a magnification of 500x to enlarge Uranus to the same size as the full Moon appears to the unaided eye.
The large outer Uranian moons Titania (magnitude +13.7, orbital period 8.7 days) and Oberon (magnitude +13.9, orbital period 13½ days) may be glimpsed in 20-cm (8-inch) and larger telescopes when at greatest elongation from their parent planet. Oberon lies 43 arcseconds (about the angular size of Jupiter at opposition) north of Uranus on 5 &18 November and a similar distance south of the planet on 11 & 24 November. Titania lies 32 arcseconds north of Uranus on 5 & 13 November and found the same distance south of its parent planet on 9 & 18 November.
* If we include asteroids, 4 Vesta at a favourable opposition comfortably makes the cut at magnitude +5.2. And during the uncomfortably close approach of near-Earth asteroid 99942 Apophis on 13 April 2029, the 450 × 170-metre rocky body could attain magnitude +3.4 as seen from a distance of just 31,000 kilometres.