A big prominence shows the early signs of instability. The four logs capture motion in the more southerly part of this large prominence. Zirin tells us "Once an arch breaks loose or rises beyond 30,000 km it will soon erupt" (Astrophysics of the Sun. p269).
30 Mm (30,000km) is not far where the sun is concerned - it's the distance covered by the cross hair e.p. in 2.8 seconds The first log (00.29UT) shows the quiet prominence, ~60Mm in height and ~230 Mm in length, already partly detached. Events moved fast - by 00:33 it was more than 30Mm above the limb, and proceeded until ended by tall trees at 00:47UT.
Note how footpoint (a) detached at 00:33 and by 00:38 was about 20Mm above the limb, while point (b) let-go around 00:38 and was fully detached by 00:47.
A SOHO TRACE coronograph showed the ensuing CME about an hour later (after emerging from behind the detector's occulting disc). One day earlier (19th) a more compact bright prominence was logged at the same site on the limb, between north latitudes +24 and +32 degrees and about 40Mm high - presumably the same prominence that remained stable until 00:33UT on the 20th.
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