⚂ Always believe in your soul
It's actually pretty hard to describe what this does without an image.
Interactive demo. Mouseover to activate, drag planets around.
This is an example of making an instance orbit around other instance in GameMaker.
In contrast to other similar examples (and commonly given advice of the kind), it allows to define orbiting speed in a common way (pixels/frame instead of degrees/frame).
This is accomplished by calculating the distance to orbit' center (from current position), calculating the orbit' length, and thus converting pixel speed to degree speed.
New position is then calculated and instance' speed is adjusted for it to reach it the next frame.
This allows to use direction in collision calculations as per usual.
Overall, the code is simple enough but documented regardless.
Code follows,
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A video.
A comparison
This post explains how to replicate GameMaker: Studio's application_surface in older versions.
Doing so is more or less a requirement if you want a scaled window with pixel-perfect rendering and/or do not desire to modify your tilesets (although a tool had been made since to help with this).
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In the example, 'notifier' is just being annoying on purpose
That's a video.