has been lengthening
-ha estado alargando
Present perfect progressive he/she conjugation of lengthen.

lengthen

For instance, 100 million years ago, the length of a day on Earth was about 22 hours, and the day has been lengthening ever since.
Por ejemplo, hace 100 millones de años, la duración de un día en la Tierra era de unos 22 horas, y el día ha estado alargando desde entonces.
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