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Solar eclipses observed at Greenwich during the time of Pond (1811-1835)

Page under construction

 

Of the eight eclipses that were potentially visible at Greenwich while Pond was Astronomer Royal, just four (1818, 1820, 1826 & 1833) are known to have been observed. Of the other four, one (1823) was recorded in the Nautical Almanac as not being visible at Greenwich, another (1832) was very minor whilst another (1813) was already underway at sunrise.

 

Eclipse Predictions visible at Greenwich by Fred Espenak and Chris O'Byrne (EclipseWise.com)
  1813-Feb-01 07:43(r)   08:15:36 09:26:28 0.618 0.521  
  1816-Nov-19 08:04:15 09:09:42 10:19:38 0.783 0.729  
** 1818-May-05 05:54:32 06:46:54 07:42:47 0.367 0.247  
** 1820-Sep-07 12:22:34 13:51:29 15:14:51 0.875 0.815  
  1823-Jul-08 05:19:36 05:32:21 05:45:11 0.03 0.006  
** 1826-Nov-29 09:47:37 10:53:02 12:00:21 0.555 0.458  
  1832-Jul-27 14:11:49 14:22:38 14:33:28 0.013 0.002  
** 1833-Jul-17 05:04:32 05:55:32 06:49:47 0.733 0.675

     (r) = already in eclipse at sunrise

     Those marked ** are the ones that were observed.

 

Eclipse predictions in the Nautical Almanac

From its inception in the 1760s, the Nautical Almanac gave predictions of the timings of any eclipse that could be seen from Greenwich. Until the volume for the year 1834, eclipse predictions were given in terms of apparent time rather than mean solar time (GMT). During Pond's period in office, they were typically given to the nearest ¼ minute. During Pond's time, the term 'Digits eclipsed' was in use. Although the term never seems to have been defined in the Almanac, in 1760, Robert Heath gave the following definition in his Astronomia Accurata; Or the Royal Astronomer and Navigator:

'The twelfth Part of the Sun's Diameter, in solar Eclipses. In lunar eclipses the Moon's Digits eclipsed may be 23: all above 12 Digits showing how much of the Earth's Shadow more than covers the Moon's nearest Edge to the Middle of that Shadow, or Eclipse.'

Digits were sometimes divided into 60 minutes ('), which themselves were occasionally divided further into 60 seconds ("). In the Almanac the symbol o rather than (d.) was normally used as the abbreviation for digit. In the volumes for 1835 onwards, the use of digits was abandoned in favour of the current term magnitude.

Eclipse predictions for the year 1818 

Eclipse predictions for the year 1820

Eclipse predictions for the year 1826

Eclipse predictions for the year 1833

 

The Observers

From 1811 until 1822 there were two Assistants at the Observatory

  • Thomas Taylor
  • John Belville (also known as John Henry)

In 1822, they were joined by two more Assistants:

  • William Richardson
  • Thomas Taylor (junior)

In 1825, two further Assistants arrived:

  • William Rogerson
  • Thomas Ellis

In 1830, Thomas Taylor Junior was replaced by

  • Frederick Simms

All of the above together with the Astronomer Royal were involved at different times in making the eclipse observations.

 

Telescopes

While Pond was Astronomer Royal, telescopes were still referred to by their length rather than their aperture. When Pond arrived at the Observatory in 1811, although it owned a number of small portable achromatic telescopes, it did not possess any fixed equatorials.

Shortly after Pond's arrival, the Observatory was offered the Shuckburgh Equatorial by the heirs of Sir George Shuckburgh Evelyn who had originally commissioned it for his own private observatory at Shuckburgh Hall. After a number of false starts it was finally ready for use at Greenwich in June 1816. Mounted in the eastern of Flamsteed’s two former summerhouses, it was known in Pond's time as both the 5-foot equatorial and the Eastern Equatorial. The location was less than ideal as nearly a quarter of the sky was obscured from view by the looming bulk of Flamsteed House to the west. To compensate, in about 1824, a second equatorial was mounted in the western summerhouse on the other side of Flamsteed House. Known as the Western Equatorial, rather than being specially made, it was cobbled together from a 30-inch telescope and the 5-foot equatorial sector that had previously been in the building.

The two most important telescopes used for eclipse observations were:

  • 46-inch achromatic, which was the only telescope to be fitted with an achromatic divided object glass micrometer
  • Shuckburgh Equatorial

Over the course of the four eclipses the following telescopes were used.

Telescope usage by year
1818
1820
1826
1833
Eclipses
5-foot achromatic Henry
 Richardson
2
46-inch achromatic Taylor
 Taylor
 Taylor
 Taylor
 4
30-inch achromatic*
 Taylor Jnr*
 Henry*
 2
30-inch achromatic* Simms* 1
42-inch achromatic Richardson 1
25-inch achromatic Rogerson 1
15-inch achromatic Rogerson 1
Shuckburgh Equatorial Pond
 Pond Ellis 3
7-foot Newtonian Henry 1
Number of observers
 2 2 6 6

* It is not known which of the observers used which of the two 30-inch acromatics

 

Observations as published in Greenwich Observations

Observations of the eclipse of 1818-May-05

Observations of the eclipse of 1820-Sep-07

Illustration from the eclipse of 1820-Sep-07

Observations of the eclipse of 1826-Nov-29

Observations of the eclipse of 1833-Jul-17, (alternative link)

 

The eclipse of 17 July 1833: notes on the published observation

The published observations

Observations of the eclipse of 1833-Jul-17, (alternative link)

Errata

Observers:

All six of the assistants, but not the Astronomer Royal observed (see table above).

Telescopes and clocks used:

The eclipse began and ended in the early hours of the morning, starting at about 5.04 and ending at about 6.50 civil time. The azimuth at the start was about 65o and at the end about 85o. Because of this, it needed to be viewed from the north side of the site. The following locations were selected: the north terrace in front of Flamsteed House, the East Dome and the Octagon Room.

On the north terrace in front of Flamsteed House, all using the same clock, which was probably a Journeyman set up there not long before the eclipse started (it was rated on the day and gained a second in just over three hours). The 46-inch was fitted with an achromatic divided object-glass micrometer:

  • 46-inch achromatic, magnifying power 75 (Taylor)
  • 30-inch achromatic, magnifying power 50 (Henry)
  • 42-inch achromatic, magnifying power 50 (Richardson)
  • 15-inch achromatic, magnifying power 35 (Rogerson)

In the East Dome and timed with the clock Arnold 1

In the Octagon Room, using the clock that was almost certainly Graham 2

  • 30-inch achromatic, magnifying power 50 (Simms)
Measurements taken
  • End of the eclipse by all observers (the start was obscured by cloud)
  • Mid point of eclipse (46-inch achromatic only)
  • Distance of the cusps as measured with a micrometer applied to the 46-inch achromatic. These were made by both Taylor and other of the assistants. Which assistants made which of the cusp measurements is not recorded in the published observations (but might be in the original manuscripts preserved at Cambridge)
  • The vertical and horizontal diameter of the Sun at the end of the eclipse with the same micrometer
  • The difference in declination between the Sun's northern limb and the point where the Moon appeared to leave the Sun with the micrometer of the 5-foot equatorial (Measurement made by Ellis rather than Simms)
Timings

The date given in the published observations is the astronomical date (16 July).

All the timings are given in terms of sidereal times as shown by the three different clocks. No attempt was been made to convert the time of the end of the eclipse into the equivalent time of the Transit Clock (Hardy), though the difference between each of the clocks and the transit clock was been noted. It is not stated how the three clocks were compared with the Transit Clock (this was probably done with a chronometer). The error of the Transit Clock is not recorded with the eclipse results. The information needed to calculate the error of the Transit Clock can be found with the Transit Observations made on the same day. No attempt was been made to convert any of the times into GMT either astronomical or civil.

There is a bit of a question mark over the rate of the clock on the terrace. At 0h32m (before the start of the eclipse) it is recorded as being 0.5s behind the transit clock and at 3h40m (after the end of the eclipse) as being 0.5s ahead of it. The same figure of 0.5s ahead of the transit clock is given for when the eclipse ended at about 2h29m23s by the terrace clock. There is no mention of a misprint in the published errata.