Month
Collier's New Encyclopedia (1921)
in astronomy, properly the time in which the moon makes one complete revolution round the earth, or appears to return to precisely the same point in the heavens from which it started. This may be from change to change, from full moon to full moon, or in an indefinite number of other ways. The time of the revolution now described is properly 29 days, 12 hours, 44 minutes, and 3 seconds. Twelve periods, called lunar months, fall short of a year by about 11 days. Lunar months were used by the ancient Jews, as they still are by their modern successors and by the Mohammedans. Also a solar month; the period for the passage of the sun through one of the signs of the zodiac. Twelve of these periods constitute a year. In law, formerly the word month in a statute meant a lunar month, but it was made to signify calendar months unless otherwise expressly designated. Anomalistic month, the time taken by the moon in passing from one perigee to the next, viz., 27 days, 13 hours, 18 minutes, and 37.4 seconds. Nodical month, the time taken by the moon in revolving from one node to the same node again, viz., 27 days, 5 hours, 5 minutes, and 36 seconds. Sacred month: suggested at Chartist Convention, London, 1838, during which the working classes were to abstain from labor to compel the government to concede the Charter. Sidereal month, the time taken by the moon in passing from one star to the same star again, viz., 27 days, 7 hours, 43 minutes, 11.5 seconds. Tropical or periodic month, the time taken by the moon in passing from any point of the ecliptic to the same point again, viz., 27 days, 7 hours, 43 minutes, 4.7 seconds.
What Has Changed in the Last Century
The numbers Collier's printed in 1921 have barely moved: the synodic month is still about 29 days, 12 hours, 44 minutes, and the sidereal month 27 days, 7 hours, 43 minutes. What has changed is how they are measured — since Apollo astronauts left laser retroreflectors on the Moon in 1969, its distance is known to within centimetres, and that work shows the Moon receding roughly 3.8 cm a year, so the month is very slowly lengthening. The lunar calendars Collier's noted are still in use: the Islamic year of twelve lunar months still runs about 11 days short of the solar year, which is why Ramadan drifts through the seasons, while the Hebrew calendar inserts a thirteenth month to stay in step. The "sacred month" of the 1838 Chartist Convention is now purely a matter of labour history, remembered as an early proposal for a general strike.
Written for this edition, 2026.