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One of Judaism's rarest rituals is performed next week, early in the morning on the eve of Pesach - the blessing on the sun. If you want to know more about it, you can read Rabbi David Hulbert's article in the current edition of the JC. The blessing is said only once every 28 years. Why 28?
Here's Rabbi Hulbert's explanation:
The source is to be found in the first tractate of the Talmud, Berachot (59b):
"Our rabbis taught that a person who sees the sun at an equinox or a solstice, the moon in her power, the planets in their orbits or the constellations of the zodiac in their order, should say ‘Blessed be He who has made the works of creation'. But when exactly? Abaye responded: every 28 years, when the Great Solar Cycle begins again at the spring equinox, when Saturn is also visible, at the beginning of the Fourth Day (ie Wednesday)."
Abaye, the head of the Pumbeditha Academy in Babylonia in the fourth century CE, was clearly a competent astronomer and well understood the motions of the sun and the planets. We moderns, for all our science, may need a few further words in order to understand this Great Solar Cycle of 28 years.
The path of the sun in the sky determines the length of the year and the cycle of the seasons. How long is a year? Like all the ancients, Abaye would have counted 365¼ days from one spring equinox (when day and night are equal in length) to the next. These 365¼ days correspond to 52 weeks, with 1¼ days left over.
So, for instance, if one spring equinox fell on Wednesday at midnight (the fourth day of creation, when the sun was created with the other heavenly bodies), next year's spring equinox will fall on Thursday at 6am, and the following year the spring equinox will be on Friday, at noon. A little arithmetic will convince you that it will take 28 years (4x7) before the spring equinox once again occurs on a Wednesday at midnight. This is the Great Solar Cycle.
Abaye would also have known that Saturn, the slowest of the five planets known to him, takes around the same 28 years to crawl around the zodiac (Saturn's year, the time that the planet takes to orbit the sun, is actually 29½ years). As it happens, if the weather is clear, Saturn will be easily visible on the night of Pesach, high in the Southern sky in Leo, above the full moon.
As we have seen, Abaye taught that we should say the blessing for the sun at the Spring Equinox, which this year took place at 11.43am on 20th March, a full 19 days before Erev Pesach. What has gone wrong? To understand the answer, we need a little more arithmetic.
The problem is that the length of the solar year, that governs the seasons, is actually not 365¼ days, but a tiny bit shorter, 365.24219 days. This discrepancy makes no difference in the short term, but over the centuries, the error has built up.
In the Christian calendar, it meant that spring started coming nearly ten days before Easter, until Pope Gregory, in 1582, wiped 10 days off the calendar and decreed a slight adjustment to the way leap years are worked out (Protestant Britain and America only made the change to the Gregorian calendar in 1752!).
The Jewish world, without a Pope or a Sanhedrin, never made the change, because our calendar is essentially lunar rather than solar, so the date we calculate for Blessing the Sun has long ago lost its tie to the Spring Equinox.