When the muscle starts to contract and needs energy, creatine phosphate transfers its phosphate back to ADP to form ATP and creatine. This reaction is catalyzed by the enzyme creatine kinase and occurs very quickly; thus, creatine phosphate-derived ATP powers the first few seconds of muscle contraction.
The point straight overhead on the celestial sphere for any observer is called the zenith and is always 90 degrees from the horizon. :) I hope this helped !