Activate timestop to get to the load zone without triggering the cutscene. This method leaves the trigger in place, so returning to this area of Termina Field will still trigger the cutscene.
This method clears the cutscene trigger so that it won't play even after returning to the area. It only works during the day.
The Astral Observatory is only meant to be accessed and left via the Bombers' Hideout, the Deku flower, and the magic bean plant. However, there are other ways in and out.
Line up in the corner as shown in the video, then recoil flip and hold right to clip. The angle is not very precise and can be set up with a shield turn instead of c-up, but whether a shield turn works depends on the notches on your controller. This clip is also possible with a HESS, but it's riskier because a HESS can clip out of bounds where Link will fall and void.
With a certain angle, it is also possible to clip with a recoil flip as described above but without holding right during the flip, making the inputs for the clip slightly easier at the cost of having to set up the angle more precisely. To do this, align the left corner of the green diamond on the in-game clock with the left edge of the grotto while in first person (not shown in video).
A goron bomb boost can get Link over the fence in either direction.