Balloon observers

[Robert Dunlop]

This issue came up up in a recent thread. Today, I was reading Walter Giffard's (an observer) diaries published in 'Guns, kites and horses' (ISBN 1 86064 906 8). On one occasion, he observed the flash of a gun that was attempting to shot him down. The balloon had to come down to be quickly repaired. He arranged for two batteries of heavies to be on standby when he went up again. 'In order to see (the German gun) really well I had to go to at least 4,000 feet as he was at least 9 miles away and behind a wood. Consequence was he put 4 over at me as I was going up, but they were poor shots and only made me doubly sure of his exact position.'

[duckman]

Hi Robert,

I have an old engineering handbook called "Chambers 7 Figure Mathematical Tables", (James Pryde (FEIS), 1947) which has a table of elevation against theoretical distance to horizon. Here are some figures from that table:

Elevation Horizon

5 ft (eye level) 2.38 nautical miles (4.4 km)

1000 ft 33.63 nautical miles (62.32 km)

2000 ft 47.56 nautical miles (88.14 km)

4000 ft 67.26 nautical miles (124.65 km)

6000 ft 82.38 nautical miles (152.67 km)

24,000 ft 166.89 nautical miles (309.29 km)

This is a purely theoretical figure based on curvature of the earth. Viewing in real conditions, allowing for atmospheric effects, rolling ground, masking by terrain (like woods) etc reduce the figures.

Just a note on the accuracy of the figures. The table takes into account the fact that air refracts light, which increases the range of the apparent horizon. My own back of the envelope calculations suggest at 24,000' the horizon would otherwise be at about 220-230 kilometres. Again, atmospheric and terrain effects mean that you won't actually see that far.

In Giffard's case, the first 100 feet would have seen him high enough to see the gun (with no intervening terrain). The other 3900 feet were so he could see into the dead ground behind the wood.

Hope that helps.

Duckman