Australian
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Teacher’s notes

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This silent black-and-white clip shows a solar eclipse recorded at Wallal in Western Australia on 21 September 1922. It shows members of the Crocker Solar Eclipse Expedition just before the eclipse, checking telescopes and other equipment and loading cameras with slides. The clip includes intertitles that refer to this activity. The final sequence shows the Moon as it moves across the Sun to obscure it and the clip then fades to black.

Educational value points

• The 1922 Crocker Solar Eclipse Expedition was funded by philanthropist William Crocker and led by Dr WW Campbell from Lick Observatory in California. The expedition is also known as the Lick Expedition. The expedition, which included Australian scientists, captured the imagination of both the public and media, who gave it extensive coverage. Expedition members gave well-attended public lectures and were feted by politicians.

• In a total solar eclipse, such as that recorded in this clip, the Moon passes between the Sun and the Earth, briefly blocking out the light from the Sun and casting a shadow on the Earth. It occurs during the full Moon phase when the Sun, Moon and Earth all line up and, while the Sun is 400 times larger than the Moon, the Moon is able to block the Sun because it is 400 times closer to the Earth. A total eclipse lasts several minutes, while nearby regions experience a partial eclipse that lasts about an hour.

• This clip may be one of the earliest moving image records of a solar eclipse and as such would have generated huge public interest. A total solar eclipse occurs about every 18 months, but because the Moon’s shadow is small compared to the size of the Earth it only affects about 1 per cent of the Earth. This means that only a small region will experience a total eclipse. This means that people may witness just one solar eclipse in their lifetime.

• The technical equipment shown in the clip would have included four cameras, each with a pair of lenses of varying apertures and focal lengths, the largest having a focal length of 40 ft (approximately 1220 cm).

• Wallal in Western Australia was selected as a site from which to observe the solar eclipse because it was directly in the path of the eclipse. The completely dark part of the shadow cast by the Moon during the eclipse is known as the umbral shadow and is visible over a path about 100 km wide.

• The Lick Expedition was ground breaking in that for the first time physicist Albert Einstein’s general theory of relativity was proven by showing that gravity can bend light. In 1915 Einstein made public his theory that space bends near massive bodies such as the Sun as a result of gravity, and that this warping of space guides objects such as planets along their orbits. A British astrophysicist, Arthur Stanley Eddington, initially tested Einstein’s theory during a solar eclipse in 1919.

• Eddington’s 1919 experiment, duplicated by the Crocker expedition, tested Einstein’s theory that space was curved by showing that light passing through space did not travel in a straight line but followed a curved path. Eddington used the total eclipse to record the position of distant stars that appear close to the Sun. A comparison of these findings with the position of the stars in the night sky at other times of the year when the Sun was further away showed that the Sun’s gravity slightly altered their position.

• As the clip indicates, by observing solar eclipses scientists can gain a greater understanding of the universe. For example, in the 18th century astronomers such as Edmund Halley monitored eclipses to determine the orbits of the Moon and Earth, and Captain James Cook used a solar eclipse in 1766 to determine the longitude of Newfoundland. An eclipse allows scientists to measure the Sun’s diameter, and also to study the physical aspects of the Sun.

• The only time that the Sun can be viewed safely with the naked eye is during a total eclipse, when the Sun is completely covered by the Moon. To look at a partial or annular eclipse or partial phases of a total solar eclipse without the proper equipment and techniques is never safe. Even when 99 per cent of the Sun’s surface is obscured during the partial phases, the remaining crescent Sun is intense enough to cause retinal burns.

• Telescopes, such as the ones shown, use glass lenses or mirrors to collect light from a distant object and focus it to form an image of the object. Originally astronomers had to record this image by hand but in the early 20th century telescopes were designed to record the image on photographic plates. Since the 1980s, telescopes have recorded images digitally. Continued advances made since the telescope was invented in the early 1600s have improved the instrument, allowing scientists to make new discoveries.

As a solar eclipse approaches, members of the scientific expedition prepare their telescopic and scientific equipment to view it. They check the lenses, position mirrors, place slides, and test shutters. Lastly, the moon’s silhouette passes across the sun. This sequence is cut with the below intertitles.

Intertitles:
As the crucial hour came round, excitement ran high and few can realise the tension of those short seconds counted out by a sailor prior to the Eclipse.

Ten minutes before the sun began to be hidden, a warning bell was struck. This was the signal for immediate activity. Slides for the camera were placed in position, shutters tested and re-tested.

Suddenly, the moon’s silhouette could be plainly seen approaching the sun, and the world gradually became enveloped in a darkening twilight.