Discovering the Outer Planets

Until 1781 Saturn was thought to be the outer edge of the solar system

The known planets M, V, M, J, S are all:
naked eye,
bright, and
easy to see

On one clear bright night in March (13th) this all changed

William Herschel (1738 - 1822) a German immigrant and professional musician / amateur astronomer thought he had found a comet.

He was systematically measuring the brightness and positions of


stars that he could view w/ a
small (?) telescope (serendipity?)


He reported it to other astronomers as a comet, but no tail appeared and the brightness remained constant

So he named it Georgium Sidus - George's Star

Why? He didn't want "On the Road Again" to remain his theme song

Other Major Works
Built big telescopes
Double (binary) stars - 1803
Clockwork Universe w/ son

Other Happenings in 1781?

Yanks beat Brits in 11th at Yorktown. French relieve Kosciusko for the save

Kant publishes Critique of Pure Reason

Family Matters

Sister Caroline his older sibling (13 yrs) - A victim of a "women's place"?

Son John - Cape of Good Hope (1834 - 1848) -star catalogs
1st person to use the word photography (1839) in print - considered a brilliant chemist

So now all the astronomers are asking . . .?

In 1821 Uranus is not where it is supposed to be

Old star charts from 1600s are discovered with Uranus on them and the positions don't match
long revolution - 84 yrs. so you now have data from 1.5 revolutions

Mathematicians have had the Principia for over a 100 yrs so you know what they've been doing - refining celestial mechanics

Yes, perturbation is rearing its ugly head.

1843 - John Couch Adams a student at Cambridge comes up with a solution that puts an unknown planet in Aquarius

He writes John Airy, the Royal Astronomer of Great Britain
and guess what?

1846 - Urbain Leverrier publishes his independent solution and guess what?

Now Airy hops to it and tells the Cambridge Observatory to search the sky at this point
and guess what?

September 23, 1846 - Johann Galle (and his proverbial student) at the Berlin Observatory are the first to observe Neptune and know it is a planet

A triumph of MATHEMATICS? I think not

Assumptions that both Adams & Leverrier made
Orbit is 2x > than Uranus (Uranus 2x> Saturn)
This means 38AU and has orbital period of 217 yrs
Eccentricity of .1 (which is large) and
at perihelion in 1820

30 AU
164 yrs
almost circular orbit ( .010 - only Venus smaller)

So the only time the model fit the reality was in 1846 + 2yrs.
and maybe it would not have been found
Remember Percival Lowell and his telescope in Tucson?

He is now searching for a new planet!

He predicts that it will be 6x> earth mass at 43 AU and it can be found in the Gemini star field.

Problem - the Gemini star field is heavy populated. He searches from 1905 until his death in 1916.

His last wish of a wide-angle telescope is fulfilled by his brother in 1927 at a cost of $10,000.

1929 Clyde Tombaugh uses the new scope with a camera
2 plates a week apart
number of stars / plate >100,000

February 18, 1930 - he finds the new planet just 6o from Lowell's predicted position

Another victory for mathematics?

Luck favors the prepared mind?