Thirteen months of 28 days
13 × 28 = 364 days. Every month is exactly four weeks, so within a year all thirteen months start on the same weekday and every date keeps its weekday from month to month.
Thirteen months of exactly 28 days. Each year begins on the spring equinox, and each month carries the name of a constellation the Sun really passes through, Ophiuchus included.
This edition pins every date and time to Eastern Time (New York), so New Year’s Day and every 13-month date are the same wherever the page is opened. The main edition uses each viewer’s own time zone.
| Body | Real sky | Astrology sign | House | Motion |
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13 × 28 = 364 days. Every month is exactly four weeks, so within a year all thirteen months start on the same weekday and every date keeps its weekday from month to month.
The March equinox is the moment the Sun crosses the equator heading north, when day and night are about equal everywhere. Day 1 is the date that moment falls on in Eastern Time (New York), the same for every viewer.
From one spring equinox to the next is about 365¼ days, so one or two days are left after month 13. They belong to no month and no week. They also replace the leap-year rule: the sky sets the length of each year.
The Sun's yearly path crosses 13 constellations, using the official borders published in 1930 by the International Astronomical Union, the body that names stars and planets. The 13th is Ophiuchus, the Serpent Bearer, which the Sun crosses for about 18 days from late November to mid-December.
Earth's axis slowly wobbles, one full turn about every 25,800 years. Astronomers call this precession. Astrology's signs were fixed about 2,000 years ago; since then the spring equinox has drifted out of Aries and into Pisces. Today the Sun is actually in Aries from about April 19 to May 14.
The constellations are uneven: the Sun spends about 44 days in Virgo and about 6½ in Scorpius. No 13 equal months can match them all. Naming the months in the sky's order, starting from Pisces, is the closest fit. Month 8 (Libra) falls almost entirely while the Sun is still in Virgo, and month 9 (Scorpius) mostly while it is still in Libra. Each month's badge shows the true count.
The Moon takes 29.53 days on average to go from one New Moon to the next, a cycle called a lunar month. A month here is 28 days, about a day and a half shorter, so each phase comes one or two days later in the month than it did the month before, except in Month 1: the Year Day or Days after Month 13 bring it back to within a day of the date it had in Month 13. About one month in twenty has no New Moon, and about one in twenty has no Full Moon. A year from one spring equinox to the next holds about 12.37 lunar months. Twelve of them make about 354.4 days, which is why a year of the Islamic calendar, 12 lunar months long, lasts about 354 days. Thirteen make about 383.9 days, close to the 383 to 385 days of a leap year in the Hebrew calendar, which adds a 13th month in 7 of every 19 years to keep pace with the seasons. So neither the regular calendar nor this one follows the Moon; its phases drift through both.
Each day shows the Moon at noon Eastern Time as it looks from the Northern Hemisphere; from the Southern Hemisphere it looks upside down, lit on the other side. Waxing means the lit part is growing, from New Moon toward Full Moon, and waning means it is shrinking. A crescent Moon is less than half lit, and a gibbous Moon is more than half lit but not full. New Moon is the moment the Moon passes the Sun in the sky, and Full Moon the moment it stands opposite the Sun. First Quarter and Last Quarter (also called third quarter) are the moments it is a quarter and three quarters of the way around. At both, the Moon is half lit: the name counts quarters of its trip, not how much of it is lit. The day that holds one of these four moments is marked New, 1st Q, Full or Last Q (with the time on screens wider than a phone), and its Moon is drawn at that moment.
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