Moonrise, Moonset & Moon Phase in Hanoi

Moon times for Hanoi are computed from the full Meeus lunar theory — the complete 47.A and 47.B periodic tables — at the city's latitude (21.0285°) and longitude (105.8542°), then converted to local time in the Asia/Bangkok timezone. Rise and set are found by sampling the moon's altitude across the day and bisecting each horizon crossing, because the moon moves about 13° a day and no single position describes it. Against US Naval Observatory tables the result agrees to the displayed minute.

Why moonrise has no fixed time. The moon travels eastward against the stars as it orbits, so the Earth has to turn a little further each day to bring it back to the horizon — about 50 minutes further. That is why a page of moon times for Hanoi cannot be written down the way sunrise times can: the cycle is 29.5 days long and does not fit the calendar. Roughly once a month a date in Hanoi has no moonrise at all, or no moonset, simply because the event slid past midnight. The calculator says so in words rather than leaving a blank.

The 18.6-year cycle. Moonrise in Hanoi is not a fixed direction. Over the moon's 18.6-year cycle the rise point sweeps 62° of horizon, from 59° ENE at its northernmost to 121° ESE at its southernmost. That is 12° wider than the sun ever manages from Hanoi, whose rise point is confined to 50° between the solstices — the moon's orbit is tilted a further 5.1° to the ecliptic, so it overshoots the sun's limits in both directions.

The moon passes directly overhead in Hanoi: its latitude falls inside the band the moon's declination covers, so at some point in the cycle the moon stands at the zenith, while at the other end it climbs no higher than 40°. Everything the moon does over Hanoi happens between those two figures, and neither of them shifts — they follow from the city's latitude and the tilt of the moon's orbit, not from the calendar.