Solar eclipse

On Wednesday morning this week, far north Queensland was treated to a total solar eclipse, while much of eastern Australia cause a glimpse of the partial eclipse. Reports from observers in Cairns reported some scattered cloud which blocked part of the totality, but the weather was fine inland and further north. The central line of the eclipse crossed the coast just south of Port Douglas. Over the course of 3.1 hours, the Moon’s umbra traveled 14,500 kilometres and covered 0.46% of Earth’s surface area.

Solar eclipse path over northern Australia on Wednesday 14 November 2012. (Credit: ASA factsheet 23v2.)

The Earth generally has two eclipse seasons a year and in each season there will be a solar and lunar eclipse two weeks apart (the former during the new moon and the latter during the full moon). The exact type of solar eclipse that occurs (total, annular, partial) depends of the precise alignment of and distance between the Sun, Earth and Moon and your location on the Earth. For solar eclipses, there is a very distinctive “eclipse path” that falls across some portion of the Earth’s surface and so whether you can see a solar eclipse from your location on the Earth depends on whether you live within that specific eclipse path or not. In the diagram above, the path represents the region of totality.

Solar eclipse can only occur when the Moon crosses the ecliptic during its orbit around the Earth. The Moon’s orbital planet around the Earth is tilted by about 5 degrees to the ecliptic.  The lunar orbital plane cross the ecliptic at the lunar line of nodes, and eclipses can only occur when the Moon crosses the line of nodes. Because the location in space of the lunar line of nodes slowly changes, eclipse seasons do not occur at the same time every year. The Moon arrives back to the same line of nodes about 18.6 days earlier in the next year. This “regression of nodes” means that the eclipse year is only about 346.62 days long (compared with a solar year which is 365.24 days long). There will be two eclipse seasons per eclipse year (also known as a draconic year).

Astronomers from all over the world crowded the beach at Palm Cove, about 20 km north of Cairns, to watch the eclipse. During a total solar eclipse, we are treated to a rare view of the solar corona. During a total solar eclipse, the incredible fine structure caused by the magnetic fields associated with active regions of the Sun are visible in the corona.  Observations of spectral lines during totality, specifically of highly ionised iron, allow astronomers to determine the composition, density variations and temperature of the corona, which is well over one million degrees – much hotter than the 5,800 K solar photosphere.

Structure in the solar corona seen during the 2006 total solar eclipse. (Credit: Fred Espenak, NASA)

As well as a team of astrophysicists attending the Eclipse on the Coral Sea conference in Palm Cove, SAO instructor Prof Karl Glazebrook and his family were there, as was SAO student Michelle Currie.  SAO alumni Sharon Harnett was just up the road at Trinity Beach.

The “diamond ring” of the 2012 solar eclipse from Palm Cove. (Credit: Karl Glazebrook, Swinburne)

Back in Melbourne, a team of PhD students at Swinburne set up a telescope to show off the partial eclipse.

Partial eclipse viewing from Swinburne University. (Credit: Paola Olivia, Swinburne)

For more details, see

[Sarah Maddison & Glen Mackie]

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