There is a proof that the Rambam agrees with the opinion of Rabbainu Tam concerning the beginning of the night. It is this: The Baal Hameor brings the language of the Geonim in the beginning of Brachot. There the language is that the Mishna says the priests can eat their truma when the firmament is clear from all light. The Rambam brings this same idea in Trumot chapter 7 halacha 2. And the sky is not pure from all light until 72 minutes after sun set. Until then, the sky gets darker, but at 58.5 minutes after sun set, there is like a half oval figure of light where the sun went down. It sinks steadily into the horizon until 72 minutes after sun set. Then the sky has become pure from all light except the stars.--------------------------------There is a proof that the רמב''ם agrees with the opinion of רבינו תם concerning the beginning f the night. It is this, the בעל המאור brings the language of the גאונים in the beginning of ברכות. There the language is that the משנה says the כהנים can eat their תרומה when the firmament is clear from all light. Rambam brings this same idea in תרומות chapter ז' halacha ב. And the sky is not pure from all light until 72 minutes after sun set. Until then, the sky gets darker, but at 58 and 1/2 minutes after sun set there is like a half oval figure of light where the sun went down. It sinks steadily into the horizon until 72 minutes after sun set. Then at 72 minutes, it has sunken entirely, and the sky has become pure from all light except the star light.