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Finding a comet is often a race. Several people may see the same faint smudge within days, and credit goes to whoever saw it first and can show it. Maria Mitchell won that race in October 1847, from a bank roof on a small island off the coast of Massachusetts. This post sets out what the records show, then uses the comet's modern orbit for a short gravitation example.
Who was Maria Mitchell?
The Maria Mitchell Association on Nantucket, which carries on her work, says she was born there on 1 August 1818 to Quaker parents. Quakers believed girls and boys should be educated alike. Her father taught her astronomy, mathematics, surveying and navigation. At 12 she helped him work out the position of their home by observing a solar eclipse. By 14, ship captains trusted her to rate their chronometers before long whaling voyages. She became the first librarian of the Nantucket Atheneum while observing at night with her father.
What happened on 1 October 1847?
The Association's account says she was sweeping the sky from the roof of the Pacific Bank on Main Street, where her father was the head cashier and the family lived. She saw a small blurry object that was not on her charts.
The independent record is a notice titled "Miss Mitchell's Comet" in the Monthly Notices of the Royal Astronomical Society for November 1847. It says the comet "was seen in America, on October 1, by Miss Maria Mitchell, of Nantucket", and observed at the Cambridge observatory in the United States on October 7. It was then seen on October 3 at Rome by Father De Vico, on October 7 by Mr Dawes in England with the naked eye, and on October 11 by Mrs Rümker at Hamburg. Dawes described it on 7 October as "a hazy star of the fifth magnitude".
So when she found it, it needed a telescope. Within days it brightened enough to see without one.
What did the discovery lead to?
The Association records that she was awarded a gold medal by the King of Denmark for the discovery, and that it made her known internationally. She became one of the first women to work for the US federal government, as computer for the ephemeris of Venus in the Nautical Almanac. In 1865 she became Professor of Astronomy at the newly founded Vassar College, and she was the first woman member of the American Academy of Arts and Sciences. She taught until 1888 and died in 1889.
How fast was the comet moving?
NASA's JPL Small-Body Database lists the comet as C/1847 T1 (Mitchell), with eccentricity 1 to the precision given, a perihelion distance of 0.329 au and an inclination of 108°. An inclination above 90° means it orbits the Sun in the opposite direction to the planets.
An orbit with eccentricity close to 1 is close to a parabola. On a parabolic orbit the speed at distance r from the Sun is the escape speed there, v = √(2GM/r).
Worked example:
Data. GM for the Sun = 1.327 × 10²⁰ m³/s². 1 au = 1.496 × 10¹¹ m, so r = 0.329 au = 4.92 × 10¹⁰ m.
Square of speed. 2GM/r = 2.654 × 10²⁰ / 4.92 × 10¹⁰ ≈ 5.39 × 10⁹ m²/s².
Speed. v ≈ 7.34 × 10⁴ m/s, about 73 km/s.
Check. At 1 au the same formula gives about 42 km/s. Since v depends on 1/√r, at 0.329 au it should be 42/√0.329 ≈ 73 km/s. It matches.
Common mistakes: using the circular-orbit formula √(GM/r), which is smaller by √2, and forgetting to convert au to metres. Our post on satellite and orbit problems covers both formulas.
How is this different from Caroline Herschel's comets?
Caroline Herschel reported comets to the Royal Society from England in the late 1700s; one of her letters appears in the Philosophical Transactions for 1787 as An account of a new comet. Our post on Caroline Herschel tells that story. Mitchell worked about sixty years later, on an island, sweeping the sky from a rooftop with a habit of regular searches.
What can a student take from how Mitchell worked?
Practise the routine. She was making a regular sweep of the sky when she found it.
Write it down at once. The date in the record is what made the discovery hers.
Use skill to serve others. Rating chronometers for whalers was applied astronomy at 14.
If the escape-speed formula still feels like magic, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Next step: find the comet's speed when it was 1.5 au from the Sun, using the same parabolic formula.
