Almost everyone in America over the age of five can tell you that Halloween is on October 31st. Ask the same people when Easter, Hanukkah, or Eid-Al-Adha are and watch them squirm as the try to define when those holidays are. These holidays change dates from one year to the next. Have you ever wondered why? Or has your child ever asked why and you didn’t know how to answer?
Mathnasium to the rescue!
To understand why some holidays change dates we must understand types of calendars. There are three main classifications of calendars: solar, lunar, and lunisolar.
Solar Calendars
A solar calendar is a calendar whose dates indicate the season, or almost equivalently, the apparent position of the sun in relation to the stars. We use a solar calendar, specifically the Gregorian calendar, to mark Halloween. In fact, most of the world uses the Gregorian calendar to mark off the dates of holidays. Easter, and all Muslim and Jewish holidays, are not tracked on a solar calendar but rather they use a lunar or lunisolar calendar. Holidays tracked on lunar and lunisolar calendars do not remain on the same date on solar calendars.
How Lunar Calendars Work
A lunar calendar tracks time by using the phases of the moon. A lunar month tracks the time from two “syzygies,” or one new moon to the next new moon. Remember that the moon doesn’t actually change shape, it just appears as various shapes because of the way it aligns with the Sun and the Earth. https://www.lpi.usra.edu/education/skytellers/moon-phases/
It takes about 29.5 days for the moon to cycle from new moon through all the phases and back to new moon again. A lunar year has 354.367 days in it. Lunar calendars have months of 29 and 30 days.
A lunar year is shorter than a solar year by eleven days.
365 days-354 days=11 day difference
The Islamic calendar is a lunar calendar. Every year the Muslim holiday Eid-Al-Adha falls at the end of Ramadan, or the ninth month in the lunar calendar. This is the month when many Muslims fast from sunrise to sunset.
Most Muslims use a solar calendar for their secular lives and the lunar calendar for their religious lives. While Ramadan and Eid-Al-Adha are consistent on the lunar calendar, they are not consistent on the solar calendar. The date change comes from the eleven-day shift that happens between the two types of calendars every year.
Lunisolar
Lunisolar calendars use both the moon phases and the seasons to track the days. The Hebrew calendar is lunisolar. The months are lunar 29 or 30 days, but the year is solar with 365 days. It has twelve months, but it often has leap months to make up the extra eleven days between the lunar and solar calendar.
Hanukkah is always celebrated on the 25th day of Kislev, the ninth month of the year. Unlike lunar calendars, lunisolar calendars do not shift compared to the solar calendar because the number of days per year are the same. However, the lunar months do not perfectly correspond to the Gregorian months. While Hanukkah is always around the same time of year, it isn’t always on the same exact date using a Gregorian calendar.
Easter and related holidays
According to many Christian scholars, Jesus’s resurrection, which is what Easter celebrates, happened on a Sunday near Passover. Passover is a Jewish holiday. Using the lunisolar Hebrew calendar, Passover is from the 15th -22 day of the lunar month Nissan. Early Christians did not always choose the same Sunday near Passover to celebrate Easter. To unify the day of celebrations church leaders voted to celebrate Easter on the first Sunday after the first full moon after the vernal equinox. In other words, to figure out when Easter will be celebrated you must pay attention to both the Sun and the Moon.
Other Calendars
It is amazing to look at the ways different cultures and eras have marked the passage of time. Although most calendars are solar, lunar, or lunisolar there have been a few other ways. Some Pre-Columbian cultures in Central America used a calendar of a fixed number of days. The Inuit, who live in arctic and sub-arctic areas, cannot track the sun and the moon the same way people in the rest of the world do. https://www.timeanddate.com/astronomy/polar-night.html In polar circles there are days in the summer when the sun is always visible and winter days where it is never visible. The Inuit developed a seasonal calendar. Their holidays mark the seasons when they should hunt animals or preserve food.
Fascinating Crossovers
Understanding how different groups of people track special occasions gives us a peek into how people use math and science in their daily lives. If you think math is just something you do in school, think again! At Mathnasium of Parker, we love math and we love to teach it in a way that makes it relevant and interesting.
Have a burning math question? Give us a call today 303-840-1184 to get more information.
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