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Cosmic Challenge: Spotting Uranus

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Cosmic Challenge: Spotting Uranus


October 2020

Phil Harrington

This month's suggested aperture range:

Naked Eye










02h 24.8m

+13° 52'




On March 12, 1781, the solar system was a simple, very well-behaved place that was best summed up with the phrase "what you see is what you get." There were the Sun, the Moon, and the five planets: Mercury, Venus, Mars, Jupiter, and Saturn. Apart from a handful of moons orbiting some of the planets and the occasional faint comet that required a telescope to be seen, the entire contents of the solar system was naked-eye territory.


Above: Autumn star map showing the location of this month's Cosmic Challenge.


Credit: Map adapted from Star Watch by Phil Harrington




Above: Finder chart for this month's Cosmic Challenge.


Credit: Chart adapted from Cosmic Challenge by Phil Harrington
Click on the chart to open a printable PDF version in a new window



All that changed on the very next night.  While looking through his homemade 6.2-inch (15.7cm) f/13.6 reflecting telescope, William Herschel stumbled onto something unexpected.  Herschel, a German-born amateur astronomer living in England, had found a greenish star that was not shown on any of his star charts.  "What a peculiar looking star," he must have thought.  In fact, if he looked carefully, he could see it was not a star at all, but rather showed a fuzzy disk.


Above: Replica of Herschel's telescope used to discover Uranus on March 13, 1781.


Returning to the same spot in the sky over the next several nights, Herschel found that his greenish discovery had moved just slightly against the background of stationary stars. The fact that the solar system had five planets was well established, leading him to believe initially that he had discovered a comet. Only after many additional observations were compiled and examined was a definitive orbit calculated. Whatever Herschel had discovered was located far beyond Saturn, the most distant planet known at the time. Further, it was following a roughly circular orbit, quite a contrast to the highly elliptical orbits of comets.


It eventually became clear that Herschel had discovered a new member of the Sun's planetary family. Herschel referred to his new find as Geogium Sidus in honor of King George III, the English monarch at the time. Fortunately, his suggestion didn't stick. If it had, however, today's school children would be learning about the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, George, and Neptune. In the end, saner heads prevailed. The name Uranus, proposed by Johann Elert Bode, was adopted by the rest of the astronomical world.


The fact that Uranus hovers near the naked-eye magnitude limit immediately raised the question in Bode's mind whether the distant planet had ever been seen by others, either with or without optical aid, before Herschel's discovery. Looking at western records only, it appears that Uranus had been seen more than 20 times prior to that fateful night in 1781, yet no observer ever recognized it as more than a faint star. That's likely due to the planet's slow solar revolution. Uranus takes just over 84 earth years to complete one voyage around the Sun.


The first recorded pre-discovery sighting of Uranus came in 1690 by John Flamsteed, the first Astronomer Royal at the Royal Greenwich Observatory. Flamsteed is most remembered for compiling his famous star catalog in which he assigned numbers to stars within each constellation in order of increasing Right Ascension. Although his catalog was initially created to help British navigators determine their longitude when sailing the high seas, astronomers still use Flamsteed's catalog to this day. While surveying Taurus on December 23, 1690, Flamsteed noted several faint stars between the Pleiades and the Hyades. He assigned one star in particular the designation 34 Tauri and moved on. As did "34 Tauri!" In fact, 22 years later, Flamsteed met "34 Tauri" again after both had progressed to Leo the Lion. Flamsteed recorded Uranus in 1712 near the star Rho (ρ) Leonis.  He also recorded it four more times in 1715, when Uranus was south of Sigma (σ) Leonis.


No fewer than three other astronomers also saw Uranus before Herschel but did not recognize it as more than a faint star.  The third Astronomer Royal, James Bradley, glimpsed it three times, first as a dim point in Capricornus in 1748, and again in Aquarius in 1750 and 1753.  Tobias Mayer, professor of economics and mathematics at Georg-August University of Göttingen, Germany, made a single observation in 1756 when Uranus was also within Aquarius.  Finally, there is the hapless tale of Pierre Charles Le Monnier, a French astronomer and physicist.  Remembered best for his temper and squabbling nature, Le Monnier apparently saw Uranus as many as ten times between the years 1764 and 1771.  Six of those observations came within a four-week window during January 1769, when Uranus had wound its way into Aries.


Above: Pre-Herschelian "discoverers" of Uranus include (left to right) John Flamsteed, James Bradley, Tobias Mayer and Pierre Charles Le Monnier.




Uranus has completed just about three orbits of the Sun since Le Monnier's time, bringing it back into Aries.  The stark backdrop of this area means that the chances are good that you will be able to single out Uranus without any optical aid provided your sky is dark and light-pollution free. To improve the odds, look for Uranus when it is near opposition and its distance away is minimal.  That occurs this year on Halloween, October 31, which makes this month the perfect time to try this naked eye challenge.  But be aware that the Full Moon will be close at hand on the 31st, so plan your attempt accordingly.


To help you in your quest, the chart above shows the current location of Uranus and surrounding field stars down to magnitude 6.5. Use binoculars to find the planet first, and then move them aside without shifting the direction of your eyes. There are no stars in its immediate area, so if you see a faint point where the symbol for Uranus () is plotted, then you got it.


Uranus passed aphelion (its furthest point from the Sun -- 1.9 billion miles or 3 billion kilometers) in February 2009. Since then, things continue to slowly improve as Uranus creeps toward perihelion (its closest point to the Sun, 1.7 billion miles or 2.7 billion kilometers) in 2050.  By then, it will have brightened modestly to magnitude 5.3.


Can you spot the seventh planet by eye alone?  I’d love to hear of your successes (or failures) in this column’s discussion forum. Or contact me directly through my web site.

Until next month, remember that half of the fun is the thrill of the chase. Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2020 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


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