Discoverers – Planets and Moons

Discoverers – Planets and Moons
Galileo's sketch after observing Neptune as a 'star' in the sky. Credit: Twitter/@Libroantiguo

The Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn are visible to the naked eye. People have seen them for thousands of years. Other Solar System bodies were discoveries, but who discovered them?

The first close-up view of the Moon through a telescope
Everyone “knows” it was Galileo Galilei in Italy in the autumn of 1609. His water color paintings of the Moon were based on his telescopic lunar observations and included in Sidereus Nuncius, the book he published in 1610.

Yet before Galileo began his Moon studies, Thomas Harriot in England had already sketched what he'd seen of the Moon through a telescope. However, he didn't publish, and it wasn't until 1784 that some of his observations appeared in print.

Galileo was ambitious, and he had family to support, so he promoted his work and was quick to go public.

Harriot had been with an expedition to the Americas where he learned the Algonquian language and studied the land and people. Although he was a mathematician and astronomer, his only published work was A Briefe and True Report of the New Found Land of Virginia.

Having received the patronage of Henry Percy, Earl of Northumberland, Harriot was established in the earl's Syon estate just outside London. There was no pressure on him to publish. Indeed he may also have chosen a low public profile when his patron ended up in the Tower of London. Thomas Percy, a cousin, was part of the Gunpowder Plot to blow up Parliament and restore a Catholic monarchy. The earl was a Protestant and not involved, but his cousin had stopped at Syon on the way to London.

Who first discovered a moon?
The Moon was a unique celestial object until the invention of the telescope. Galileo saw three objects near Jupiter on January 7, 1610, and later a fourth. Observing over several nights, he concluded they were not distant stars, but were moons orbiting Jupiter. His observations later featured in his book Sidereus Nuncius.

In 1614 German astronomer Simon Marius published Mundus Iovialis in which he reported details of his discovery of the Jovian moons. The first sighting was on January 8, 1610. Galileo was outraged, calling Marius a liar and a plagiarist. It seems that Marius was neither. But it took three hundred years to validate his claim when a panel of experts in the Netherlands concluded that he had made an independent discovery. Part of the evidence was the detailed tables of his observations. Marius himself acknowledged Galileo's precedence, but that didn't mollify Galileo.

Marius followed Johannes Kepler's suggestion of naming the moons after Jupiter's lovers Io, Europa, Ganymede and Callisto. Although it didn't catch on immediately, those are the names now in use.

Seeing isn't discovering
Uranus is often described as the first planet to be discovered telescopically. Yet in the right conditions, Uranus can be seen with the naked eye. And John Flamsteed, the first Astronomer Royal at Greenwich, saw Uranus several times with his telescope, starting in December 1690. He catalogued it as the star 34 Tauri.

Even the man who discovered Uranus in March 1781 – William Herschel – didn't recognize it as a planet. His telescope was good enough to show him that it wasn't a star, but it probably never occurred to him that it was a new planet. He reported it as a comet. Nevil Maskelyne, then the Astronomer Royal, was one of the first to suggest that Herschel's discovery was a new planet.

Who discovered the first asteroid?
The Titius-Bode Law was a formula that predicted the distance of each planet from the Sun. When Uranus was discovered, its distance also followed the formula. All that was missing was a planet at 2.8 AU. (The Astronomical Unit is the Earth-Sun distance.)

On New Year's Day 1801, Sicilian astronomer Giuseppe Piazzi discovered at 2.77 AU the body he named Ceres. Here was the missing planet! Ceres and the next three bodies discovered in similar orbits were all listed as planets. But they were later classified as a new class of object. Herschel called them asteroids because they didn't show a disk like a planet, but seemed to be points of light like a star.

Piazzi had discovered the first asteroid. In the 21st century it was also classified as a dwarf planet, and visited by a NASA probe.

Who first saw Neptune?
Johann Galle gets the credit for finding Neptune, but he did so based on the calculations Urbain Le Verrier sent him. Neptune is often described as the first planet to be discovered mathematically.

Although Galle was the first person to know that he was looking at a new planet, he was not the first person to see it. One previous observer was John Herschel, son of William Herschel. In noting it as a faint star, he missed the chance of a great family double.

Neptune is a telescopic object, so no one could have seen it without a telescope. Interestingly, it appears that one of the very first people who could have seen Neptune did see it. It was Galileo in December 1612. His drawings of Jupiter with its moons include a “star” that was actually Neptune.

How can we be confident about what Galileo saw four centuries ago? Planetarium software can show us the sky on the nights of Galileo's dated sketches. Here is Galileo's sketch on the night of December 27, 1612, with Jupiter and its moons and a "background star". Starry Night software shows us that the "star" is Neptune.



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