Great show for the kids...but be warned.
Tickets for the planetarium shows (and other shows like the lightening show) are on sale when science works opens aka at 10am.
These tickets sell fast...very very fast.
You cannot pre-order or buy tickets for these shows.
Be at the science works before opening (by 1/2hr at least if not earlier) and line up to ensure you get "Show" tickets.
i have found this the hard way...arrived at 10am...all tickets gone by 10.05am!

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When you have your show tickets wander around the rest of the science works...Try to get tickets for the lightening show as well.
Edit:
are we talking about the planetarium at the Melbourne science works?