By Carol Redford
On average, there are two to three solar eclipses every year.
The first thing to understand is that there are two main types of solar eclipses – central and partial. In both types, the Moon passes between the Sun and Earth. In a central eclipse, the Sun, Moon, and Earth line up nearly perfectly, such that the Moon’s centre crosses in front of the Sun’s. In a partial eclipse, the alignment isn’t as straight, and the Moon just appears to “takes a bite” out of the Sun, anything from a small nibble to a big chomp.
Both the Earth’s orbit around the Sun and the Moon’s orbit around Earth are elliptical, or slightly oval, which means that when the Moon passes directly in front of the Sun as seen from Earth, it can appear at any size from slightly smaller to slightly larger than the Sun. This produces two main types of central solar eclipses:
Total Solar Eclipse
When the Moon appears larger than the Sun, we see a total eclipse, when daylight gives way to deep twilight and the spectacularly beautiful solar corona is revealed.
Annual Solar Eclipse
When the Moon appears smaller than the Sun, we see an annular (ring) eclipse, leaving a brilliant “ring of fire” around the Moon’s dark silhouette.
What is a Hybrid (Annular-Total) Solar Eclipse?
Sometimes, a central solar eclipse can be annular in some places and total in others.
Throughout an eclipse, the Moon is moving along in its orbit. Its inner shadow (from within which you can see totality or annularity) traces a long, narrow path along Earth’s surface. Earth’s surface, being curved with a radius of about 6,500kms, means that a person standing near the centre of Earth’s blue-white face (as seen from the Sun or Moon) is about 6,500kms closer than a person standing near the limb (edge).
If the Sun and Moon are very nearly exactly the same size as seen from Earth, it is possible that the Moon will not quite cover the Sun at either end of the path (annular), but will cover it at the middle of the path (total). This is what we call a hybrid, or annular-total, solar eclipse.
The next total solar eclipse visible from Western Australia in on 20 April 2023.
The 20 April 2023 solar eclipse will begin and end as an annular eclipse. In Exmouth, the eclipse will be total. The Moon will appear only very slightly bigger than the Sun from Exmouth and totality won’t last long – only about a minute. Observers will likely see a broken ring of the Sun’s chromosphere, the ruby-red atmospheric layer between the photosphere (bright face) and the corona. The chromosphere and corona can be viewed without eye protection, and the colour contrast between the black silhouette of the Moon, the red chromosphere, the pearly white corona, and the twilight-blue sky is about as magnificent a sight as you’ll ever see!
WARNING: Never look at the Sun without special eye protection except during the total phase of a total eclipse. Whenever part or all of the Sun’s bright face is showing, you must use a special-purpose safe solar filter, that is, one that complies with the Australian Standard AS ISO 12312-2:2020. Dark sunglasses are not nearly dark enough for safe solar viewing.
Astronomical details above kindly provided by Rick Fienberg, American Astronomical Society.
Further technical details on the 2023 hybrid, or annular-total, solar eclipse are available from www.EclipseWise.com.
What does this Total Solar Eclipse mean for WA’s Tourism Industry? Visit Astrotourism WA to find out more…