Transit of Mercury
Tomorrow between 5am and 10am Australian Eastern Standard Time (+1 hr if you’re on daylight saving), there will be a Transit of Mercury - where Mercury passes between the Earth and the Sun. Similar to a solar eclipse, when the moon goes between the Earth and Sun, Mercury will block out part of the sun’s light. Unlike a lunar eclipse, however, Mercury appears (and really is!) much smaller than the sun in the sky (about 1/200th of the size!) and so will appear as a small black dot moving across the face of the sun over about 5 hours. (Even though the moon and sun appear as almost exactly the same size in the sky, this is just a neat coincidence - the sun is actually much larger, but just enough farther away from the Earth to compensate and be able to almost exactly cover up the sun during a total eclipse.)
You can see an animation of what the transit will actually look like (you might need to hit Shift+Reload to see the image play again - I did, anyway). Because Mercury is so much smaller, you won’t be able to see this transit without a small telescope (not that you should be looking at the sun directly, anyway!) You should project the image from the telescope onto a piece of white card, and Mercury should be visible. Never, NEVER look at the sun directly through the telescope - you’ll fry your eyes, literally. UNLESS you’re lucky enough to have a sun filter - that blocks out most of the light, and you can observe the sun directly (also great for seeing sunspots and even solar flares!) There’s a good PDF factsheet with some more details of the transit and the setup you’ll need to observe it.
There are also various places around Australia (and beyond!) where you can observe the transit with professional astronomers - search on Google! If you live in Brisbane, come to the roof of the Parnell Building (Bldg 7) of the University of Queensland (just keep finding stairs that go up!) and we’ll have three telescopes set up - all welcome! Now, just cross your fingers for clear skies, because although transits of Mercury happen every 5-7 years, this will be the last one visible from Australia until 2032!
Beyond the intrinsic coolness factor, transits of Mercury and Venus help us better analyse planets and learn about their atmospheres (none of the other planets can have transits, since they orbit farther away from the sun than Earth!) It’s also by looking for transits of planets in front of distant stars that we have discovered many extrasolar planets - planets in other solar systems! Oh, and my final piece of trivia comes from ABC Science - it was a transit of Mercury which, ultimately, lead to the discovery of Australia by Britain - an observation of a transit of Mercury inspired Edmond Halley to suggest that transits could be used to determine distances between planets in the solar system, which lead to Captain Cook’s voyage to Taihiti to see a transit of Venus, which in turn lead to the British colonisation of Australia!