April 16-19, 2022: Mercury passes Uranus in the evening sky. Use a binocular to find the pair in the west-northwest after sunset.
by Jeffrey L. Hunt
The planet Uranus was first thought to be a comet, when it was spotted by William Herschel on March 13, 1781. Through his telescope, he saw a tiny globe, without the characteristic tail. He thought this comet was closer to the sun than Earth.
Over a month later, Herschel’s discovery was announced in a scientific paper before Britain’s esteemed Royal Society entitled, “Account of a Comet.”
Other observers across Europe pointed their telescopes skyward at the new “comet” in the constellation Gemini. Initially, they agreed with Herschel that this was a comet, but it continued to show a globe without a cometary tail or other comet-like appearance.
By the summer of 1781, it was concluded that this was not a comet, but a new planet. Initial calculations indicated that it was at least twelve times Earth’s solar distance. As the observation refined the planet’s movement, the distance was recalculated to be 19 times Earth’s distance.
The next year, Herschel was named King George III’s court astronomer. The astronomer suggested that the new world be named after the king. Other astronomers suggested Neptune.
Johann Bode, the era’s unofficial arbiter of celestial names, suggested Uranus to keep the mythological genealogy for the planet names. In mythology, Uranus was married to Earth. Father Sky was the paternal origin for other major mythological characters that have planets named for them.
Bode looked at other astronomers’ records to find that the planet had first been recorded in 1690 in the constellation Taurus.
Uranus is not easily seen with the unaided eye. It is near the limit of eyesight and in modern cities and suburbs, the brightly-lit night makes it impossible to see without optical assistance. The presence of another planet or the moon calls our attention to it. Through a binocular it has a color of aquamarine and the globe is visible through a small telescope with modest magnifications.
The accepted solar distance is 19.2 times Earth’s distance and it revolves around the sun every 84 years, nearly three times Saturn’s plodding speed.
During the next few evenings, Uranus and Mercury are in the same binocular field. This is not an easy observation as they are near each other during mid-twilight. Uranus is less than 7° above the west-northwest horizon.
The accompanying chart shows the planets April 16-19 as Mercury speeds past Uranus.
Uranus is near the star Omicron Arietis (ο Ari on the chart). On April 16, Mercury is 2.7° to the lower right of Uranus. On the next evening, Mercury is 2.0° to the upper right of the distant world.
On April 18, Mercury is farther away, 2.6° to the upper right and near the star Pi Arietis (π Ari). On the final night of suggested observations, Mercury, near the star Rho Arietis (ρ Ari), is nearly 3.8° above Uranus.
It should be noted that the pair might be visible together through the binocular as early as April 14 and as late as April 21. During the earlier dates, Mercury is very low in the sky, while the same occurs for Uranus on April 20 and 21.
Uranus dips into bright twilight and reaches its solar conjunction on May 5th. Then it moves into the morning sky. Venus passes the planet on June 12.
For those who want more history about the first observations of Uranus, read any general textbook about astronomy. Morton Gosser’s The Discovery of Neptune recounts the history of the first observations of the two large outer planets. The book is out of print from the original publisher, but it is available on online resale stores.
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