Planets and Transits

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In the face of the Sun
Planets and Transits

• A new view of the Universe III
• Fred Watson
• April 2005

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What is a transit?
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Transit geometry
Why isnt there a Venus transit every year or
so? Venus orbit is tilted relative to Earths
This makes transits of Venus jolly rare events
Planets and Transits
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Cyclic phenomenon
Transits of Venus follow a 243-year repeating
pattern. They always occur in June or December
because that is when the Earth crosses the line
of nodes.
Planets and Transits
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The dawn of the telescope era
Della Portas sketch of 1609 shows the principle.
But it was Galileo who turned it into an engine
of discovery in the same year
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By the way, while were talking about the birth
of the telescope
To be published in the US September by Da Capo
Put it on your Christmas list TODAY!
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The 1639 transit
Drat! Ive missed the good bit.
Predicted by Jeremiah Horrocks (1618-1641).
Observed by him at Much Hoole, 4 Dec. 1639. Had
to dash off to church before the event
started. Venus was in mid-transit when he
returned.
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What he actually said
Then I beheld a most agreeable spectacle a spot
of unusualmagnitude and of a perfectly circular
shape
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The 1639 transit
It was also observed by his friend William
Crabtree (1610-1644), a Manchester
cloth-merchant.
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More transit geometry
Why is a Venus transit potentially so useful? It
allows you to measure the distance to the Sun
Bright idea courtesy of Edmond Halley
(1656-1742)
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The 1769 transit
By then, telescopes had improved significantly.
And governments were prepared to fund expeditions
to far-flung places to get the best view of the
transit.
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The 1769 transit

• James Cook in Tahiti
• (And the rest is history)

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The trouble with transits
Cook and others found their timings spoiled by
the black-drop effect...
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The 1874 transit in Australia
Henry C. Russell Never perhaps in the worlds
history did morning dawn on so many waiting
astronomers as it did on 9 Dec. 1874. He
observed from Sydney, Woodford, Eden and Goulburn.
Russells rival, John Tebbutt, disparagingly
described his report on the 1874 transit as a
gorgeous volume
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And again in 1882
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Consternation!
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Photographic assistance
Taken by William Harkness of the US Naval
Observatory, 6 Dec 1882. (It was cloudy in
Sydney.)
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Harkness looks to the future
We are now on the eve of the second transit of a
pair, after which there will be no other till the
twenty-first century of our era has dawned upon
the earth, and the June flowers are blooming in
2004 What will be the state of science when the
next transit season arrives, God only
knows. (Dec 1882.)
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The transit of June 2012
Dont look directly at the Sun (especially not
through binoculars or a telescope)! Use eyepiece
projection (like Horrocks) Or check webcasts from
NASA and other agencies (unlike Horrocks).
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Planets and Transits
But what actually is a planet?
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Isnt it obvious? Its one of these
In fact, its a serious contemporary problem.
Or these
Or maybe one of these
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Is it a star?
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An object that shines only by reflected light?
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An object that orbits the Sun?
To detect Jupiter need 12.5 m/s accuracy. For
Saturn 2.7 m/s. Attainable accuracy 2 m/s.
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An object that orbits the Sun?
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An object that orbits a star?
Free-floating objects in Orion with masses less
than 13 Jupiters
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An object big enough for its own gravity to crush
it into a spherical shape?
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And what about Pluto compared with Kuiper belt
objects?
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The astronomical community wrestles with the
definition of a planet
How are planets distinct from moons, asteroids,
brown dwarfs, stars? Response of the
International Astronomical Union (the only body
that can make the definition) is Well, er Or,
to be more precise 1) Objects orbiting around
solar-type stars with true masses above the
limiting mass for thermonuclear fusion of
deuterium (currently calculated to be 13 Jupiter
masses for objects of solar metallicity) are
"brown dwarfs" (no matter how they formed) while
objects with true masses below this limiting mass
are "planets". 2) Free-floating objects in young
star clusters (which presumably formed in the
same manner as stars and have not been shown to
be ejected from planetary systems) with masses
below the limiting mass for thermonuclear fusion
of deuterium are not "planets", but are
"sub-brown dwarfs" (or whatever name is most
appropriate).
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What is the use of transits today?
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So, looking for the characteristic dimming is a
good way to find extra-solar planets
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And it might tell us much about the the planet
and star, as well as how many are out there
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Hence new telescopes

• such as the 0.9-m RCT (Robotically Controlled
  Telescope) at Kitt Peak in Arizona.

Jeremiah Horrocks would have thought this was a
pretty neat way to see planetary transits...
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In the face of the Sun Planets and Transits THE
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