Re: historical astronomy question

[Rick Strickert <rick_strickert@URSCORP.COM>]

Brian Whatcott noted:

> (He [Reverend Teddy Reece Phillips, sometime secretary
> and president of RAS] gives the date as Dec 4th 1639
> by the way).

The December 4th date (modern calendar) for the 1639 transit of Venus is
correct, as I found out from two other references:

1. David Sellers,(http://www.dsellers.demon.co.uk/venus/ven_ch4.htm), gives
the 1631 transit date as December 6th (modern calendar) and the 1639
transit date as December 6th. BTW, Sellers published a bedtime reading
favorite (;-)), _Hidden Beneath Our Feet: The Story of Sewerage in Leeds_
(Leeds City Council, Department of Highways and Transportation, December
1997, 40 pages, ISBN 0 902384 24 4, price £4).

2. J. Donald Fernie, ("Transits, Travels, and Tribulations", American
Scientist, March-April 1997, pp.120-122,
http://www.sigmaxi.org/amsci/other/Fernie/marginalia9703.html) also notes
the transit date in 1631 was December 6th (predicted by Kepler in 1627).
Fernie also states that Horrocks reworked Kepler's calculation to discover
only a few weeks before it was to take place, that another transit would
take place in 1639.   According to Fernie, Kepler thought it would be a
near miss (Kepler died in 1630).

The web site, http://www.btinternet.com/~redlion/jerry.htm, is probably
based on Allan Chapman's, "Jeremiah Horrocks, the Transit of Venus, and the
'New Astronomy' in early seventeenth-century England". Qtrly. Jrnl. Ry.
Astr. Soc, 31 (1990), pp 333-357.

I don't know whether Chapman or the person who prepared the web site
entered the wrong modern date for the 1639 transit and I don't have
immediate access to the Quarterly Journal of the Royal Astronomical
Society.

BTW, according to Chapman, Horrocks started to observe the transit of Venus
at 3.15 p.m., when the Sun was within half an hour of setting.  During the
thirty minutes of sunlight that remained, he made three accurate
measurements, as the black dot of Venus moved across the Sun's face.

Chapman claims, "That half-hour was one of the most momentous in British
astronomy."  By his transit observations, Horrocks accomplished the
following:

1. measured the angular size of the planet Venus;
2. calculated the beginning and ending times of the transit;
3. calculate the planet's orbital velocity;
4. determine important information about the orbit of Venus;
5. along with subsequent measurements, worked to prove Venus
     had an elliptical orbit; and
6. was the first major achievement of British astronomical research.


Not bad for a half-hour's work!   ;-)


Rick Strickert
URS Radian
Austin, TX