Time is one of the slipperiest concepts in all of science. Everyone feels that they know what time is, but when we try to measure it, we quickly run into difficulties. For early scientists and astronomers, the sky itself served as the first clock and calendar.

The sky above us is constantly changing and full of wonderful objects that never stop moving! As scientists and astronomers, one of our first tasks is to be able to say when and where something interesting happened. The ability to locate things in time and space, both in an absolute sense, and in relation to one another, is a fundamental skill. In this unit, we will explore measuring the Earth-Moon system with time, and then move on to show how science can accommodate different ideas and explanations for the same observations! Only experiments can tell us which model is correct!

Activity 7: The Earth Clock

The concept of time is intimately connected with astronomy, and more particularly with the spinning Earth. We divide the Earth into 24 time zones, it takes the Sun one hour to move across each one of these zones.

The motion of the sundial’s shadow around the gnomon gives is the ‘clockwise’ direction (turning to the right). This motion is also intimately related to the Earth’s spinning motion on its axis.

In today’s world of digital clocks and cell phones, the concept of a 24 hour day being related to the rotation of the Earth has become more remote. This activity will bring home to your very modern students that the old fashioned idea of the sundial and the spinning Earth are closely connected with the time we keep.

For the Educator

Facts you need to know

1. The Earth is both our oldest, and one of the most accurate clocks, spinning each day in exactly 24 hours (86,400 seconds!)
2. Diurnal motion is the daily motion we see as the Sun and Moon rise in the east and cross the sky to set in the west. This is also apparent motion, caused by the rapid spinning of the Earth on its axis – not by any actual movement of the Sun or Moon in space.
3. Unlike our Sun which rises consistently at about the same time each day, the Moon’s rising and setting time changes, rising and setting by almost an hour later each day.
4. The time of moonrise and moonset are intimately tied to the Moon’s orbital motion around the Earth.

Teaching and Pedagogy

The concept that the measurement of time is linked to the daily motion of the Sun across the sky is a very ancient one. The Sun and Moon are the brightest and most obvious things in our sky and their regular motions and changes make them a natural focus for time keeping. Civilizations around the world have universally developed solar and lunar calendars in their earliest pre-history.

More than 2,200 years ago, a Greek named Aristarchus of Samos came up with the first known heliocentric model of the solar system. In a time when most educated people believed that the Sun revolved around the Earth every day, Aristarchus theorized that a spinning Earth and a stationary Sun would explain the same diurnal motion we see in the sky each day as the Sun rises, crosses the sky from east to west, and then sets again.

Most people see, but do not reflect upon the diurnal motions of the Sun and Moon. It is a difficult thing at first, to lift your perception from off the surface of the Earth and envision the motion of the Earth as it spins upon its axis and revolves in orbit around the Sun. The best thing about this activity is that it helps the student extend their perception and envision our world as a planet in orbit around a star.

When we teach these activities to our students, we must take care to help the student see the larger picture. When we help students see beyond the ball and string of the model and make a connection to our solar system and how it works, these changes in perception can be both effective and lasting.

Sometimes in science, we have competing theories that both try to explain the same thing. We can argue if we wish, but only time, and careful experiments, can settle the issue for good! For older students in 5^th grade and up, you may wish to show both theories with your activity. First have the Sun orbit slowly around the Earth which stands still. From the point of view of our Earth observer, the Sun will still rise in the East and set in the West at the correct times each day. After that, do the activity as described above – the Earth observer will see the same motion of the Sun across the sky!

I do not recommend showing competing theories to younger students however, as it can promote misconceptions and be confusing to them!

Students Outcomes

Conducting the Activity

Materials

1. A large (3-ft) piece of cardboard – a science fair poster board works well for this.
2. A set of irrigation flags
3. An old baseball cap (adjustable size works best.)
4. Wooden yardstick
5. A large ball to serve as the Sun
6. A yellow vinyl play ball is preferred, but a basketball or soccer ball may be used easily enough.
7. Several 2-ft pieces of rope or strong cord (clothesline cord works well)
8. Markers or paints
9. Construction paper – various colors (optional)
10. Hot glue gun

Building the Earth Clock Model

1. [Teacher] Begin by hot gluing the yardstick horizontally across the back of the large piece of cardboard. This keeps the cardboard ridged and makes it more durable. If you are using a folding piece of cardboard such as a science fair poster board, you can attach the yardsticks with Velcro. This will insure the cardboard piece is still foldable and stores more easily.
2. [Teacher] Using a screwdriver, punch two holes in the cardboard (one above the yardstick, one below) at each end of the yardstick. Thread a 2-ft piece of rope or cord through the holes and knot it securely on the yardstick side. Use hot glue to secure the rope in place. This creates handle loops to help students hold onto the device.
3. Take two irrigation flags and mark them as East and West (you may also use index cards for this.) Use duct tape to attach them firmly to the back of the artificial horizon so the flag sticks up over the edge of the cardboard and is visible to everyone. When looking at the front (smooth side) of the artificial horizon, the East flag goes on the right side, while the West flag goes on the left side.
4. [Optional] Students can decorate the horizon by adding a skyline at the eastern and western edges. These can be drawn on poster board and then cut out and taped or glued in place. This allows the person using the horizon to see the Moon in relation to houses, mountains, etc.
5. Make a ‘Time Hat’ by cutting out a long arrow (12-15 inches long) from poster board and taping or gluing it to the top of the hat so that the arrow points straight out past the center of the bill of the hat.
6. Mark 12 irrigation flags as follows: 2 am, 4 am, 6 am, 8 am, 10 am, Noon, 2 pm, 4 pm, 6 pm, 8 pm, 10 pm, and Midnight. If you have different color flags, use one color for am and another color for pm. Alternatively, you can staple two different colors of construction paper to the flags and mark them that way. The flags work well in any grassy area.

7. Place an irrigation flag in the grass to mark the center of your clock face. Use a cord as a compass (the 7-ft cord from the Earth-Moon system model works well) and mark out a clock face on the ground using the labeled irrigation flags to show the hours. Remember that you are marking a 24-hour clock, so instead of having 12, 3, 6, and 9 at the cardinal points like a standard clock face, you will have Noon, 6 pm, Midnight, and 6 am. Place the other hour markers appropriately.

Exploring the Earth Clock Model

1. With your clock face marked out, half of the circle represents AM (daytime) and half of the circle represents PM (night time). Have a student hold the Sun ball at the Noon position. All is now ready!
2. The student playing Earth must hold the artificial horizon cardboard steadily across their shoulders (rather like a backpack!). The horizon limits their view to 180 degrees (just like the real horizon does) and prevents them from looking behind themselves (we cannot see ‘behind’ the planet, either!)
3. Begin standing facing the Sun, and the Noon flag. Whichever flag they are facing tells the time (they are the hour hand on our clock!) The first ‘day’ begins at noon with the Sun directly overhead!
4. The Earth student now spins slowly to the left (anti-clockwise) – this represents the Earth’s daily rotation on its axis. As they turn slowly, they will see the Sun move slowly westward, and finally disappear over the western horizon! What time is it? The Earth clock will say approximately 6 pm. The student may object that they are moving, not the Sun – Exactly!
5. Continuing to spin to the left, the student will see the Sun rise again over the eastern horizon – they will now be facing the 6 am flag – sunrise! Have each student spin through several days so that everyone gets the concept of the diurnal motion of the Sun – and understands that it is caused by the spinning motion of the Earth and that the Sun does not actually move at all!

Supplemental Materials

Going Deeper

We are all familiar with the idea of the leap year, when we add a day to the calendar every four years. We add this extra day because the Earth’s orbit around the Sun takes 365.26 days. We have to deal with the extra quarter day by adding a day to our calendar every four years. In effect, we use the leap year to clean up messy fractions that wouldn’t work in our calendar!

An interesting variation on this idea is the leap second. Like the leap year, this idea is used to clean up messy fractions. We say that the Earth’s day is exactly 24 hours or 84,600 seconds, but in fact this is not true! Like the Earth’s rotation around the Sun, the Earth’s spin on its axis does not match our clocks and calendars precisely.

Explore the idea of the leap second; search the internet and see what you find.

1. Is the Earth rotation time shorter or longer than 84,600 seconds? By how much?
2. Is there a regular schedule for adding a leap second? (Remember the leap year happens on a regular schedule every four years.)

Being an Astronomer

Timing the rising of the Sun or Moon can be a reasonable way to time the Earth’s rotation! This works best when sunrise or moonrise is straight up off the horizon; for this reason you will get the most accurate results timing the sunrise in June, and the moonrise in December. All this requires is a stopwatch!

Position yourself to see the Sun or Moon rise over a flat edge – the edge of a building works well, students can watch the Sun come up over the roof of their own house on a clear morning!

Start timing when you can first see the edge of the Sun’s disk, and stop when the disk is completely over the edge and clear of the building; this will take about two minutes. Remember that the Sun is blindingly bright – don’t stare at the solar disk the whole time, just glance at it occasionally so you know when to stop your timer!

Take the time in seconds and multiply by 720^[1]. The Earth’s actual rotation period is 86,400 seconds (24 hours) – how close did you get?

Being a Scientist

When we think about what a day is, most people think about the time between sunrise and sunset. The problem is that the number of hours of daylight we have changes throughout the year, this is also part of our Earth Clock.

An interesting investigation can be made by graphing the number of hours of daylight for every day of the year. Students can do this by using an app or website to tell them how many hours of daylight each day; or by using a weather website to find the time of sunrise and sunset and working out how many hours each day and then plotting the results on a daily graph.

The graph should look something like this:

Plot the length of the day in hours on the 1^st, 7^th, 14^th, and 21^st of each month. Over the length of the year you should see a beautiful curve formed by the points on your graph. 12 hours is used as the center point of the graph because that represents a day perfectly divided with equal hours of daylight and darkness. These days are called the equinoxes; the name comes from the Latin language, meaning equal night. See how many equinox days you can find in a year.

There are also days when we have the longest and shortest day; these days are called solstices. The word solstice also comes from the Latin, meaning Sun stands still. Can you find the longest and shortest days of the year on the graph? How do these days relate to the seasons? How can we explain these slow and steady changes of daylight and darkness? We will explore these ideas further later in the book!

Following Up

Having a regular place in your classroom where you record days of the week or showing the month and date is fairly common in a classroom. These things help students develop their sense of time, seasons, weeks, semesters, etc. Consider adding some astronomical features to your daily calendar such as the phase of the Moon, the length of the day, or noting equinox and solstice days!

Activity 8: Moonrise and Moonset

This is a fascinating activity for young and old alike. Everyone is aware that the Sun rises early each morning, the time changes a bit from season to season, but sunrise is remarkably consistent. Moonrise is no such thing! Many people know that the Moon is sometimes visible in the early morning sky, but few people take note that the Moon rises about an hour later each day. If the time of sunrise is so consistent, why is the time of moonrise so variable? This activity answers this question with an exciting ballet of planetary and orbital motion that is sure to inspire everyone in your class!

For the Educator

Facts you need to know

1. We all know that the Earth spins on its axis and the Moon orbits the Earth – but most people don’t think about these two motions occurring at the same time.
2. Each time the Earth turns once on its axis (one day), the Moon has moved in its orbit.
3. Because of the Moon’s motion, the Earth has to turn a bit more than 360 degrees to see the Moon rise over the horizon each day. This change accounts for the changing times of moonrise each day.

Teaching and Pedagogy

This activity is a complex ballet that involves almost everyone in the classroom. With younger students, you may have to practice the different parts of the activity separately before you can pull the whole thing off; doing activity #5 first will be crucial for them!

It is also important to help students understand that what the person in the center in the Earth position sees is what we all see from here on Earth. Both the daily apparent motion (diurnal motion) and the more gradual orbital motion of the Moon should be apparent to them as they participate in the activity.

Don’t worry if the very youngest students don’t completely catch on to the entire scientific significance of the activity with all its subtlety! Introducing students to a scientifically accurate concept when they are young will help these ideas to ‘click!’ when they see them again in a year or two when they are older and more sophisticated thinkers!

Student Outcomes

Conducting the Activity

Materials

1. Artificial horizon (See activity #7)
2. A set of irrigation flags with clock hours on them (See activity #7)
3. Sidewalk chalk (for pavement), or 30 unmarked irrigation flags (for a lawn) to mark out the Moon’s orbit
4. Sun model – a 12-inch yellow vinyl play ball is preferred ($3), but any soccer or basketball will do.
5. Moon model – a 12-inch vinyl play ball – dark blue or black is preferred, but you can paint any color ball half-black, half-white for this.
6. Ten, 12-inch squares of poster board (construction paper or cardboard may be used)
7. A can of flat-black spray paint
8. A can of flat-white spray paint
9. Markers or paints

Building the Moonrise and Moonset Model

1. Take seven, 12-inch squares of poster board and mark then with large numerals 1-7. If you do not have a separate ball for your Sun model, draw and label a large Sun on another piece of poster board.
2. [Teacher] Make a Moon model by masking off half of your dark-colored vinyl play ball with masking tape and newspaper. Prop the ball on an empty soup can and spray paint half the ball flat white. Let the ball dry completely before handling it.
   • Note: If the paint on your model does not dry properly, dust it liberally with corn starch and let it sit overnight. Brush off the corn starch with a dry paint brush and your model will be perfectly dry and ready to use!
3. Now take all the pieces of your model outdoors and choose a place on the lawn or playground for the Earth and mark it with chalk or an irrigation flag. Have one student start at the Earth position, and walk two steps away. Stretch a piece of string between the Earth position and this student. Using this string as a compass, mark out the face of the clock, starting with Noon. Remember that this is a 24-hour clock face! Instead of 12, 3, 6, and 9 o’clock, we will have Noon, 6pm, Midnight, and 6am at the cardinal points.
4. Have a student start at the Earth position and walk 4-½ steps away – this is the distance to the Moon’s orbit. Stretch a string between the Earth position and this student as a compass. Mark out the path of the Moon’s orbit with sidewalk chalk if on pavement, or with a series of irrigation flags about 2 ft. apart if you are on a lawn.
5. Have a student hold the Sun model well outside the Moon’s orbit in the Noon position. This will allow the students to see the Moon both in the evening and morning if you continue the Moon’s orbit long enough!
6. One student will hold the Moon model, also starting in the Noon position. Remind them to keep the white portion of the Moon pointing in the same direction at all times! With the Moon in this position, the student in the Earth position will see ‘new moon’ – none of the white portion of your Moon model will be visible.
7. One student will now play the Earth – they get to wear the Time Hat you have prepared! Have this student use the rope loops to hold the artificial horizon against their back (rather like a backpack!) while standing at the center of the circle. Start the student off facing the noon flag – remember to emphasize that the student in the Earth position is the hour hand of the clock – whichever flag is straight ahead of them – that’s what time it is on the Earth Clock!
8. Have a student stand just outside the lunar orbit holding up the “Day 1” poster board to mark the Moon’s first position. The stage is now set, time to set Earth and Moon in motion!

Exploring the Moonrise and Moonset Model

1. As the Earth turns slowly anti-clockwise in place (revolving on its axis!), have the Earth student look to their right (over the western horizon). Tell them to stop when they can no longer see the Moon – this is moonset! The ‘Earth’ can now look straight ahead – the arrow on the Time Hat will now point to the correct time of moonset! (This will be about 6pm.)
2. As the Earth continues to spin, the Moon moves one step anti-clockwise around its orbit^[2], and another student will mark the position by holding up the poster board denoting the number of the new day.
3. Point out to your students that the spinning Earth will now have to turn just a bit farther than 360-degrees to see the Moon over the eastern horizon again – this is moonrise. When they reach the point where they can see the Moon again – check the Earth clock – it should show about 7 pm. Moonrise has changed by about an hour!
4. Have the ‘Earth’ take note of the Moon’s phase at moonrise on the second day – if the bright side of the Moon has been held in a steady direction, they will see a thin crescent moon!
5. By continuing to advance the Moon each day, everyone can see that the Moon is moving from west to east in its orbit, making moonrise and moonset time about an hour later each day. But the student playing Earth will see something else – as they spin slowly to the left (eastward!), the Moon will rise over the eastern horizon, and travel across the sky (their field of vision) and set in the west. Each day will also see the Moon’s phase increase, the crescent will gradually increase to quarter phase, and then gibbous and full if you continue the activity long enough.
6. Allow as many students as possible to take the Earth position and try this out. There is nothing like being at the center of things to improve your perspective and understand cognitively and kinesthetically that the Earth’s spin creates the east to west motion we see each day, and the Moon’s orbital motion creates the west to east motion that we see over days and weeks.

Supplemental Materials

Going Deeper

1. Aristotle said the Earth was fixed; he believed that the Earth was immobile, it neither spun on its axis nor orbited around the Sun. In fact, Aristotle believed that the Earth didn’t move though space at all, and his models dominated scientific thinking for almost 2000 years! Use the internet to find some the ancient scientific explanations Aristotle used to try and convince people that the Earth did not move or spin, can you explain why these are not true using what you have learned in these activities?
2. Making an accurate clock was an important scientific quest for many centuries! In fact, scientists today are still striving to make ever more accurate clocks! Can you think of a way to make an accurate clock? Can you build one? [Hint: Start your students looking at pendulums and old-fashioned grandfather clocks. They may also want to investigate Galileo and his water clock!]

Being an Astronomer

It is time to be a backyard astronomer again and take another look at the Moon! Start at the new moon phase and watch over a series of nights to see where the Moon appears at sunset. Watching the Moon at the same time each day will be important for the success of this activity!

Students can use irrigation flags, or even just sticks or small rocks to note where the Moon appears over the horizon each night. Place one flag to mark your observing spot, stand in this same place each night. Standing in your chosen spot, point to the position of the Moon at sunset. Take a 6-foot piece of string and stretch it across the ground and use a flag or stone to mark the direction in which you see the Moon. A parent can help with this!

Over the course of several nights, you will note that the position of the Moon in the sky at sunset moves steadily from west to east! Our scientific model of the Moon’s orbit is confirmed! If the student or parent has a smart phone, take a photo of the diagram you’ve created after a week or so of observations to show what you have discovered!

Being a Scientist

Scientists often gather data to detect patterns in Nature; you can do this with the Moon as well. For this activity, it is important to have a consistent – and safe! – from which to watch the Moon each night. One easy way to do this is if you have a window that looks to the west; this keeps you inside safe and warm! The best time to do this is just after new moon. This means the Moon will be visible in the western sky just after sunset.

Watch the Moon set into the west and record the time when the Moon is no longer visible. This may be when the Moon drops below the horizon, or when it goes behind a building; as long as you use the same point of reference each night your experiment will work fine.

Keep in mind that the Moon sets later each night, you will only be able to get three or perhaps four nights before moonset is too late for you to stay up!

Record the time of moonset each night. After you have finished collecting several days of data, do the math to figure out how many minutes of change you observed in moonset each day.

Our activity predicts a change of about 51 minutes change each day. Can your observations confirm this? How close did you get to this figure?

Following Up

Have you been keeping track on your whiteboard of things like the phase of the Moon and hours of daylight along with the date and day of the week? This can be a great time to add a new feature: tracking the Moon’s position in orbit around the Earth.

Make a set of ‘orbital magnets’ by coloring small circles of cardboard – one yellow for the Sun, one blue for the Earth, and a grey one for the Moon. You can move the Moon around the Earth, changing its position 2-3 times each week. Remember that during one entire week (7 days), the Moon must move 90 degrees in orbit.