Educator How-To: We’ll See You on the Dark (and Light and Far) Side of the Moon

Guess what time it is? Yup, you guessed it. It’s time for another installment of my Educator How-Tos! This week, we’re going celestial with a phases of the moon demonstration.

The moon over HMNS' George Observatory.

The phases of the moon is an important concept that students must understand. After years of experience, we here at the Museum feel we have refined this activity to the point that it can be taught and understood easily.

Step One. Determine what your audience “thinks” they know. We need them to understand the following before we begin:

  • The moon orbits earth.
  • Sunlight reflects off the surface of the moon to create brightness.
  • The moon appears to change shape over time, and we call this change “the phases of the moon.”

Some misconceptions you may encounter:

  • The moon produces its own light, like the Sun. No, the moon reflects sunlight, and that is why it is illuminated.
  • Earth’s shadow explains the phases of the moon. This is an eclipse and is a completely different event.
  • The moon does not rotate. This is incorrect. For our purposes, the moon will not rotate in the demonstration, but the moon does indeed rotate, albeit more slowly than earth. The moon does not appear to rotate because we always see the same side. However, the reason we always see the same side is because it rotates exactly one time during each orbit around the earth.
  • People confuse the dark side of the moon and the far side of the moon. There is indeed a dark side, and a light side for that matter, but this is a cyclical event with alternating sides of the moon being dark and light at different times. The far side of the moon, however, is the side of the moon that we never see from our vantage point on earth.

Step Two. Ask your audience if they can name the phases of the moon. For our purposes today, we will only be considering the full moon, half moon (quarter moon), crescent moon, and new moon.

Special Note: The half moon is traditionally called the quarter moon, but I find it confusing, so I use the term half moon instead. You may handle this challenge in a way that works best for you. Both terms are correct.

Step Three. Explain this key concept before you begin the demonstration: There is always half of the moon that is in darkness and half that is in light at all times — regardless of the portion of the lighted moon that is visible to us on Earth. This can easily be explained by calling the dark side “nighttime” on the moon, and the light side “daytime.” Just as on earth, when one side is experiencing daytime, the other side is always experiencing night time.

Demonstration. Now you will demonstrate this concept using a bright light source (an overhead projector works well), a blow up Moon (or another light-colored ball that is large) and a volunteer.

The key concepts determined in Step One (light reflects off the moon as the moon orbits the Earth and the moon appears to change shape) are modeled in this demonstration. Use the following steps to demonstrate moon phases:

  • Pick a volunteer from the audience.
  • Tell the class that the volunteer is going to represent our perspective of the “moon” from different angles. They should envision viewing the “moon” through the volunteer’s eyes.
  • Have the volunteer stand directly to the left of the light source (which is representative of the Sun).
  • The instructor, holding the “moon”, should stand directly in front of the “sun” (about four to six feet away). Now that everyone is in the proper place, you may begin.
  • The volunteer, looking directly at the “moon” should describe the amount of light (or darkness) they observe. From this perspective the entire visible portion of the “moon” should be in the light. This is representative of a full moon. When one observes a full moon they are viewing the entire half of the Moon that is in full daylight.
  • Have the volunteer move 90 degrees to the left. Again ask them to describe what portion of the visible “moon” is in the light (or dark). From this new perspective, half of the visible portion of the “moon” should be illuminated. This is a half moon (quarter moon). When one observes a half moon they are only seeing half of the moon that is in actual daylight — the rest of the illuminated portion is not visible from an earthbound perspective.
  • The volunteer should move 45 degrees to the left. Again, ask them to describe what portion of the “moon” is in the light (or dark). From this new perspective 1/4 of the visible portion of the “moon” should be illuminated. This is what is called a crescent moon. When one observes a crescent moon, they are only viewing ¼ of the Moon that is in actual daylight — the rest is not visible from an earthbound perspective.
  • The volunteer should now move an additional 45 degrees to the left. The volunteer will now be standing opposite the instructor and the “moon”. Again, ask them to describe what portion of the “moon” is in the light (or dark). From this new perspective, none of the visible portion of the “moon” should be illuminated. This is what is called a new moon. When one observes a new moon, they observing the half that is in “nighttime,” but remember — the daytime side is still on the other side.

Viewpoints & Phases of the Moon

Key Point Summary:

  • The moon is always half daytime and half nighttime.
  • As the moon orbits the Earth, our perspective changes and we see different amounts of light and dark area.
  • The appearance of the change in shape is just a change in the amount of lighted side visible from Earth.

Great Caesar’s Ghost!

Creative Commons License photo credit: glacial23

Why is this month called July? The short answer is that July is named for Julius Caesar. The longer answer involves the ancient Romans’ attempt to keep track of the year.

There are two bright lights in the sky which can help us mark the passage of a year: the Sun and the Moon. Since the year is defined by the Earth’s motion around the Sun, it is best to measure it using the Sun’s apparent changing position among the stars. However, this observation is difficult, since we do not see the Sun and the background stars at the same time. The first ancient culture to do this was ancient Egypt, where the rising of Sirius just before the Sun occurred right before the annual flood of the Nile. Egyptians marked the beginning of each new year with this event.

Sirius vanishes for several months each year when Earth’s orbital motion puts the swath of sky containing Sirius on the far side of the Sun, and therefore behind the Sun from our point of view. Eventually, Earth’s continued motion brings that swath of sky from behind the Sun, allowing Sirius to rise just before dawn. To measure years by the reappearance of Sirius, then, is to measure them by the positions of the Earth and the Sun–a solar calendar.

Creative Commons License photo credit: ComputerHotline

The Moon, by contrast, is much easier to observe each clear night. Most ancient cultures, including the ancient Romans, measured months by tracking the cycle of lunar phases from new to full and back to new. Since a lunar phase cycle takes 29.5 days on average, we might expect the month lengths to alternate between 30 and 29 days. However, the Romans considered even numbers unfortunate, so their months, beginning with March, had the following lengths:

Martius 31, Aprilis 29, Maius 31, Junius 29, Quintilis 31, Sextilis 29, September 29, October 31, November 29, December 29, Januarius 29, Februarius 28

Yes, January and February were tacked on to the end of the year at first (the Romans originally did not count days at all between December and March) and later moved to the beginning. (In 153 BC, Roman consuls began to take office as of January 1, making that the start of the civil year). February was given an even number of days (making it unfortunate) so that the year as a whole would be fortunate, with an odd number of days.

Unfortunately, there is not an even number of lunar phase cycles per solar year. A cycle of 12 lunar months is 354 days long, 11 days shorter than the cycle of seasons which is about 365.25 days long. (The Roman calendar outlined above has 355 days, because the Romans preferred odd numbers). If this is not corrected, each month occurs 11 days earlier each year, compared to the start of the seasons, and months are no longer associated with the seasons. This is exactly what happens in the Muslim calendar. To keep months roughly aligned with the seasons, an extra 13th lunar month must be added to some years, as in the Hebrew calendar. Thus, the Romans periodically added an extra month, called Intercalaris, between February and March. After all, February was originally at the end of the year.

Martes 13
But is it really? Ordinary Romans didn’t
Creative Commons License photo credit: kozumel

However, it was up to the Roman priests to decide which years would have the extra month. Priests often used this power to arbitrarily shorten the terms of political opponents and lengthen the terms of their friends. Also, the extra month was considered unlucky and avoided in times of crisis, such as the Second Punic War against Carthage. By Caesar’s time the calendar had become so chaotic that regular Roman citizens, especially those far from Rome, did not know the date.

Even as he waged civil war against his rivals for power in Rome, Caesar began many popular reforms as dictator. He offered citizenship to many more Romans and enlarged the Senate to provide more representation. He canceled one fourth of all debts. And he reformed the calendar so even the average Roman could know the date.

While in Egypt, he consulted the Greek astronomer Sosigenes, who told him of the Egyptian solar year of 365 days. Caesar thus decided to add ten days to the 355 day Roman calendar. First, in 46 BC, he realigned the months with their traditional seasons by using Intercalaris and by adding two more months between November and December. 46 BC thus became a 445-day year, the ‘last year of confusion.’ Beginning in 45 BC, the new 365 day Julian Calendar was to come into effect. Caesar added 2 days each to January, ‘Sextilis’, and December, and one day each to April, June, September, and November. The original long months (March, May, Quintilis, and October) remained 31 days long. February, the ‘unlucky’ month devoted to religious rituals, also remained unchanged.

The extra month Intercalaris was dropped forever, replaced by an extra leap day every four years. As Romans had added the extra month in late February, Ceasar placed leap day there as well. The year took its modern shape starting in 45 BC:

January 31, February 28 (29), March 31, April 30, May 31, June 30, Quintilis 31, Sextilis 31, September 30, October 31, November 30, December 31

Julius Caesar
Creative Commons License photo credit:
get directly down

On the Ides of March (March 15), 44 BC, Julius Caesar was assassinated. The Roman Senate felt that a fitting tribute, given Caesar’s work with the calendar, would be to name a month of the year after him. Caesar had been born on the 4th day to the Ides of Quintilis (Quintilis 12th). When that month came around in 44 BC, the Senate proclaimed that from that year on, Quintilis shall be known as Julius (or July in English).

Romans at first counted inclusively, i.e. 1, 2, 3, 4/1, 2, 3, 4/1, 2, 3…. This led them to use leap years too often. The first Roman emperor, Caesar Augustus, noticed this and suppressed all leap years between 9 BC and AD 8. In recognition of this, the Senate offered Augustus a month, and he chose Sextilis, the month when his greatest victories had occurred. Thus Sextilis became August.

As it turns out, the year is not exactly 365.25 days long; its closer to 365.2422 days. The Julian year is thus 11.8 minutes too long on average. There is some evidence that Caesar, Sosigenes et al. knew of the error but considered it insignificant. However, those 11.8 minutes add up to 1 day about every 130 years. In 1582, Pope Gregory XIII noticed that the year had gotten off by 10 days since AD 325, when the Nicene council set the rule for calculating the date of Easter. The Pope decreed that October 15 would follow October 4, 1582, and that century years are leap years only if they are divisible by 400 (thus 2000 was a leap year, but 2100, 2200, and 2300 will not be). Our current calendar, then, is the Gregorian calendar, not the Julian.

But consider this: Pope Gregory did not change the length of the year, nor the lengths of the months, nor the fact the the leap day is in February. All of these decisions by Julius Caesar remain in effect. Caesar, then, still deserves his place of honor in the year.

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