Science of Holidays

Author: Arlene Vinion Dubiel

The holiday season is upon us. It’s a time for giving gifts, good food, and celebrating with family and friends. We decorate our houses with greenery, light candles, and remember times past. No matter your religious background or lack thereof, you are likely celebrating something this season. Why do so many holidays happen this time of year? And why is it that Christmas is a consistent December 25 but Hanukkah, Diwali, and other holidays change dates? How are these dates determined?

Turns out, there is a natural reason for this holiday season. In this blog, we will explore the science behind this time of year and how different cultures celebrate this natural phenomenon.

Seasons

Imagine you are a human living about 4,000 years ago in what is now England. You have simple tools, fire to light and warm the night, and little else. You depend upon nature to provide what you need to survive – food from plants and animals, shelter from the ground or trees, and warmth from animal skins. The passage of time is marked by the location of the sun in the sky from sunrise to sunset. You notice that as days go by, time of sunlight becomes shorter. Less sun means less heat. It also means fewer live, tasty plants and more competition with other animals for these plants. Eventually, the night is much longer than the day, which is a cause for worry because if this trend continues, you will freeze or starve. So, you and your fellow humans start to appeal to a higher power to bring back the sun. You go through various rituals and rites and slowly, the sun comes back. You notice that this pattern for the sun is predictable, year after year, so you and your fellow humans develop a large calendar of sorts and use it as a center for worship and reverence. Millennia later, other humans will find your calendar and call it Stonehenge.   

The natural cycle for the seasons is predictable and is based upon the relationship between the Earth and the sun. We know that winter is cold with long nights and summer is hot with long days. But why do we have summer and winter? What are the true reasons for the seasons?

In the famous video “A Private Universe,” Harvard graduates attempt to explain the reasons for seasons. Most of them explain how the Earth is closer to the Sun in the summer and farther from the sun in the winter. The video then follows Heather Mars, a middle school student, as she explains the reasons for seasons while having her naïve conceptions revealed and corrected. The video is a call to dive into students’ conceptions and correct them as needed.   

How do we correct for students’ naïve conceptions? One way is to have students face information that contradicts their conceptions. For example, I wonder what would happen if the Harvard graduates in “A Private Universe” were asked if their explanation fits with the fact that the Southern hemisphere has summer at the same time that the Northern hemisphere has winter. Or if they were asked about daylight hours during the summer and winter.

Revealing these naïve conceptions about seasons and subsequently building an accurate model of the Earth and sun is the crux of one of my favorite resources for middle school science teachers: “The Real Reasons for Seasons” by Lawrence Hall of Science. This guide provides solid evidence to dispel the naïve conception that the distance between the Earth and the Sun can have a significant effect on seasons. For example, did you know that the Earth is closest to the sun in January? And if you track the number of hours of daylight around the world, you would see that the arctic circle has 24 hours of daylight in June while Antarctica has zero and locations around the equator have 12 hours of daylight no matter the month. 

Average daily temperatures align with the number of hours of daylight for locations around the globe. The more daylight, the higher the temperature. The Earth is tilted on its axis by 23.5 degrees in relation to the sun. We know this because the Earth’s north pole points towards the North Star or Polaris. When you take a time-lapse image of the night sky, Polaris does not move. Because of this tilt in relation to the sun, there are varying hours of daylight depending upon latitude. The more hours and the more concentrated the sun’s rays on the ground, the more heat and light there is. And this is why we have summer and winter in the different hemispheres and why there is a lack of seasons for locations near the equator. . 

“The Real Reasons for Seasons” takes students through multiple activities to fully build a model of the Earth and sun relationships. This is a core idea for middle school students according to the Next Generation Science Standards. Standard MS-ESS1.1 states “Students who demonstrate understanding can: develop and use a model of the Earth-sun-moon system to describe the cyclic patterns of lunar phases, eclipses of the sun and moon, and seasons.” While I’ve tried to explain the full reasons for seasons here, it is much more fun to go through the activities and it’s much more likely you will remember and have a deeper understanding of the reasons. If you do teach middle school science or Earth science, I encourage you to check out “The Real Reasons for Seasons.”

Holidays and seasons

In the northern hemisphere, the fewest number of daylight hours occurs at the winter solstice which is on or around December 21 according to our modern Gregorian calendar. Many ancient and modern holidays that feature light as a theme with candles, bonfires, lanterns, fireworks, and more are celebrated around this time when the day is short and the night is long. 

Yule is an ancient holiday season celebrated by Pagans. One particular ritual is the lighting of the Yule log – a large log placed on the fire on the night of the winter solstice celebrating the return of the sun in the next days. Saturnalia is the Pagan holiday celebrated by Romans. Traditions include decorating homes with greenery and gifting candles to symbolize the return of the sun. These holidays are usually multi-day celebrations with the pinnacle on the longest night of the winter solstice.

Christmas is the Christian holiday celebrating the birth of Jesus. While many theologians argue that Jesus was born in the spring, Christmas is celebrated on December 25. It is thought that this was to usurp the Pagan holidays that occur during this time of year. Indeed, many traditions of Christmas are borrowed from the Pagan celebrations and people who are not Christian still celebrate Christmas with indoor trees, gift-giving, and candles in windows.

Kwanza was first celebrated on December 26, 1966 by African Americans. This week-long celebration focuses on moral values and the goodness of life and features gift-giving and the lighting of candles. The timing of Kwanza is linked to many traditional African celebrations of the first harvest. The idea of the first harvest celebration in December makes sense if you consider that parts of Africa are in the southern hemisphere where spring with all of its first fruits is coming to an end and summer is beginning.

The holidays of Christmas, Kwanza, and the pagan celebrations have consistent dates on our Gregorian calendar. Other holidays near the winter solstice with light as a theme vary depending on the year. These holidays include Hanukkah, Diwali, and Chinese New Year. The cultures from which these holidays come do not use the Gregorian calendar, but calendars that have their basis on the phases of the moon in addition to the sun and seasons.   

Moon cycles

It takes 29.5 days for the moon to complete a full cycle of phases – called a synodic period. The cycle starts at the new moon and slowly we see more of the moon lit until the full moon when it cycles back to the new moon. The period between new moon and full, the moon is waxing and the period between full and new phases, the moon is waning.

Phases of the moon are traditionally introduced in 4th grade. This is appropriate as students in 4th grade are also introduced to angles in mathematics. Phases are also taught in middle school as indicated with the Standard MS-ESS1.1, mentioned above, but is not covered in high school. Because moon phases are taught so early and not covered again, many adults harbor naïve conceptions which we will discuss here. 

The sun is the only thing in our solar system that produces light. All planets and our moon reflect the light that comes from the sun. One common naïve conception is that the phases of the moon are a result of the Earth casting a shadow on the moon. This conception does not account for the shape of the gibbous moon nor for the regular pattern of moonrise and moonset. The only time shadow of the Earth comes into play is when there is a lunar eclipse – the full moon passes through the Earth’s shadow. The next total lunar eclipse is on May 26, 2021.

Instead, the phase of the moon is due to the angle between the sun and moon with the vertex being our observation point here on Earth. When the angle is 180°, we see a full moon. When it is 0°, we do not see any light reflected and the phase is new. At 90°, the moon appears to be half-lit. Note that if we were to go far into outer space and look at our moon, it would always be half-lit from the sun, as would the Earth causing our days and nights. This phenomenon is best modeled with a light bulb, a ball on a stick, and then imagining your head as the Earth. Earth, Moon, and Stars LHS GEMS Teacher’s Guide explains this phenomenon well with students having to first observe the moon in nature for a period of time and then building a model based upon the patterns observed. With these observations, it is possible to see the patterns of moonrise and moonset as related to sunrise and sunset. Once you know these patterns, you can sometimes catch naïve conceptions about the moon in popular media. For example, the full moon rises at sunset and is only directly overhead at midnight. A thin crescent moon is never visible high in the night sky. Rather it is close to the sun so is best viewed just after sunset on the western horizon if it is waxing and just before sunrise on the eastern horizon if it is waning. 

I mentioned before that our modern Gregorian calendar is based upon the sun’s position in our sky. Other calendars are based upon both the sun and the moon. It takes just over 365 days for the Earth to revolve around the sun and there are 354 days in 12 lunar cycles. With this discrepancy, it takes 19 years or 235 lunar months for the phases of the moon to be in sync with the solar cycle. In other words, there will be a new moon on December 15, 2020 and the next new moon on December 15th will be 19 years from now in 2039. This period of time of 235 lunar months is called the Metonic cycle. 

Holidays and moon phases

Hanukkah is the Jewish celebration of the rededication of the temple after the Maccabean revolt in the second century B.C.E. It lasts eight days and begins on the 25th day of the month of Kislev and ends on the 2nd or 3rd of Tevet. Because the Jewish calendar is based upon the Metonic cycle, the date of the start of Hanukkah varies on the Gregorian calendar. This year, 2020, Hanukkah starts at sundown on December 10. The timing of this holiday coincides with both the long nights close to the winter solstice and with the new moon so it is not surprising that this celebration features the lighting of candles on the menorah.

Diwali is a festival of lights that usually occurs in the Gregorian Calendar months of October or November. In 2020, it began on November 14. The timing of this festival is based upon the solar-lunar Hindu calendar. This festival commemorates the return of Lord Rama from his years in exile and celebrates the triumph over spiritual darkness. As you may expect, the timing of this festival also coincides with the new moon.  

The first new moon in Aquarius is when Chinese cultures celebrate the lunar New Year. This is another celebration of light with lanterns and fireworks to light the night sky as the moon is not visible. The Chinese calendar is based upon the moon phases and the timing of this festival, also called the Spring festival, starts on the 23rd day of the 12th lunar month and lasts several days into the new year. The next Chinese New Year starts on February 12, 2021.

 With all of these holidays involving light happening around the winter solstice, with some also coinciding with the new moon, it makes me wonder whether the Aboriginal peoples of Australia have similar celebrations around what we call the summer solstice. Likewise, I wonder if there are any celebrations of light among the native peoples that live near the equator where there is almost a constant 12 hours of daylight. Regardless of your traditions or celebrations this time of year, enjoy the long and dark nights with your own light and know that the sun will return soon. 

Happy Holidays!

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