Want to see Uranus for yourself? Independence Day morning offers one of the best opportunities in decades, as the distant ice giant passes extraordinarily close to Mars in the predawn sky.
Although we often hear that only five planets are visible to the unaided eye, Uranus can also be seen from Earth under the right conditions. As the seventh planet from the sun, it is very faint — near the threshold of naked-eye visibility at roughly sixth magnitude — so viewing it requires a very dark sky with little to no significant light pollution.
The second challenge is knowing exactly where to look. Uranus is faint enough to blend into a background of similarly dim stars, but early on Saturday, July 4, at around 4 a.m. local daylight time, Mars provides a useful guide. On that Independence Day morning, Mars and Uranus will appear unusually close together in the sky.
Finding Mars first
Mars currently rises about half an hour before the first light of dawn. It is still relatively faint at about magnitude +1.3, and through a telescope it appears tiny, measuring less than 4.5 arc seconds across. Look for it low in the east-northeast, about 5.5 degrees directly below the Pleiades star cluster.
- Find Mars low in the east-northeast before dawn.
- Point binoculars or a small telescope at Mars.
- Look almost directly above Mars for a much fainter, star-like point of light. That will be Uranus.
What Uranus will look like
Uranus should resemble a tiny greenish star and will appear only about 1/63 as bright as Mars. Although Uranus is nearly 7.5 times larger than Mars, it is more than 9.5 times farther away as seen from Earth. At about 1.88 billion miles (3.02 billion km), it appears only slightly smaller than Mars in apparent size, measuring about 3.5 arc seconds across.
How close will they be?
According to Belgian celestial calculator Jean Meeus, Mars and Uranus will be closest at 5 a.m. Universal Time (UTC) on July 4, when only 6 arc minutes will separate them. For comparison, Mizar — the middle star in the Big Dipper‘s handle — and its fainter companion Alcor are separated by about 12 arc minutes. In other words, Mars and Uranus will appear only half as far apart as Mizar and Alcor.
Observers with exceptionally sharp vision and very dark skies may want to try spotting Uranus near Mars without optical aid, though binoculars or a small telescope will make the view much easier.
Mars and Uranus are in conjunction on average once every 2.38 years. But meetings as close as this one are quite rare, occurring on average once about every 40 years. The next similarly close approach between these two worlds that is readily visible in a dark sky is not due until Dec. 8, 2147!
A Bonus Target: HIP 19146
If you are using a small telescope, look for an even fainter object about 2 arc minutes below Uranus. This is not a Uranian satellite, but a background star cataloged as HIP 19146. The “HIP” designation comes from Hipparcos; a European Space Agency satellite launched in 1989 and operated until 1993. Hipparcos was the first space mission devoted to precision astrometry, the accurate measurement of celestial positions and distances.
On the morning of the upcoming Mars-Uranus conjunction, try to spot this eighth-magnitude star as well. It is about 11 times dimmer than Uranus and lies roughly 882 light-years from Earth.
Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer’s Almanac and other publications.
