Around this time of year, Halloween, people often think about all things spooky and supernatural, which some refer to as the occult. Well, the occult also applies to astronomy (not just because we work at night).
The word occult comes from words meaning "secret" or "to hide", and this is what occult means in an astronomical sense - it is when one object briefly passes in front of another and hides it.
For example, when there is a total solar eclipse, and the moon blocks the light from the sun, this is an occultation. However, when we see a lunar eclipse, with the Earth's shadow covering the moon, that isn't an occultation.
We learnt about the solar corona, the outermost layer of the sun, and where much of the solar wind comes from by observing what can still be seen during a solar eclipse. The rest of the time, the sun is just too bright for us to see the corona, unless we use special filters space telescopes.
If the whole occultation process can be observed, or a series of occultations by one object over time, then we can learn about what is doing the blocking; how big it is, how fast it is going, and in which direction.
Using the occult is how many asteroids have been found - the asteroid passes in front of stars in the field of view, blocking their light, but only for a short time. If the whole occultation process can be observed, or a series of occultations by one object over time, then we can learn about what is doing the blocking; how big it is, how fast it is going, and in which direction.
We can even learn more about planets when they occult distant stars - for example, the rings of Uranus were discovered in 1977 when it occulted a star (although Saturn is famous for its rings, it isn't the only planet that has them, all the giant planets do too).
Even Pluto has occulted a few stars, and the light from the background stars allowed us to learn about its atmosphere before the arrival of the New Horizons spacecraft in July 2015.
On the more extreme end of occultations, there is gravitational lensing, a phenomenon where a large body (like a cluster of galaxies) passes in front of background objects and bends the light from them to create a warped image.
Gravitational lensing can help us to better understand the background objects that are being lensed, because it can create a magnifying effect.
We even use a similar method to detect exoplanets, planets around distant stars.
When a star with a planet passes across a background star, the overall light is changed by both the presence of the occulting star and the occulting planet.
This method of exoplanet detection requires a lot of preparation and planning on the part of the astronomer.
Using this kind of occultation to find exoplanets is called microlensing, and it is gravitational lensing on a much smaller scale.
Occultations are a very useful tool in astronomy, so maybe there is something to learn from the occult.
- Eloise Birchall has a Masters of Astronomy and Astrophysics (Advanced) from the Australian National University.
