Download A Human-Powered Orrery: Connecting Learners with the Night Sky*

A Human-Powered Orrery: Connecting Learners with the Night Sky*
Larry Lebofsky, Michelle Higgins, Don McCarthy, Nancy Lebofsky, and the NIRCam E/PO Team
Introduction
Visualizing planetary motions and their relationships to each other is difficult for many learners. In many of
our outreach programs over the years, we modeled the motion of the Earth around the Sun and the seasonal
constellations, but this did not involve a lot of audience participation. Based on a series of articles and
presentations, we finally came up with the idea of a portable human orrery that allowed for greater audience
participation.
An orrery is a mechanical device that shows the relative positions of the planets in our Solar System. The
first orrery was probably built in the first century BCE. One of the first modern orreries was constructed by
John Rowley for Charles Boyle, the 4th Earl of Orrery hence the name we use today. While most orreries are
mechanical (See Fig. 1), modern orreries can also be computer-generated. Orreries can be fairly simple, they
can be works of craftsmanship, or they can be made with Lego® pieces! Unfortunately, while illustrating the
relative positions of the planets, orreries also create many misconceptions: the relative sizes of the planets,
the relative distances of the planets from the Sun, the causes of the phases of the Moon, etc. They are not
intended to be accurate scale models of the Solar System.
Figure 1: Four examples of mechanical orreries Far left, orrery for sale (credit: www.1worldglobes.com); left, replica
of James Furguson’s orrery for sale (credit: www.curiousminds.co.uk); right, an orrery showing the inner planets
(credit: wikimedia commons); far right, an orrery made with Lego® pieces, built by George Moody (credit:
ecg.mit.edu/George//lego/orrery.html/)
Background on Human Orreries
Figure 2: Armagh Observatory
orrery (Credit: Armagh Observatory)
There are several “human orreries” that show the correct relative
scale distances between the planets and give participants the
opportunity to become the planets as they orbit the Sun. There are at
least two permanent human orreries, one in Dynic Astropark in Japan
and another at the Armagh Observatory, Northern Ireland (Astronomy
and Physics News 2005, 46:3.31–3.35; Sky and Telescope May 2005;
http://www.arm.ac.uk/orrery/). The Armagh Observatory model even
has elliptical orbits and is able to show the correct relative positions
of the planets.
*This project was supported in part by a SEED grant from the Astronomical Society of the Pacific
(www.astrosociety.org). Please contact Larry Lebofsky ([email protected]) if you have any questions.
Human-Powered Orrery 4-15-15.docx
1
Since then, two activities have appeared that can be assembled and demonstrated: a GEMS activity
(http://kepler.nasa.gov/files/mws/HumanOrrerySSSmsGEMS.pdf) designed for middle school, linked from
the Kepler website, and one designed for a college-level class in Vancouver, Canada (Peter Newbury, BAAS
41:660; The Physics Teacher 48:473). A YouTube video of this activity can be found at:
http://www.youtube.com/watch?v=ju4cfEp2BgU
Because the last two models are limited to a one-hour class period, including setup, there is little that is
intended for the participants to experience from these activities beyond the relative distances and motions
and, perhaps, the relative sizes of the planets. While there is much that can be investigated with the
permanent orreries, the models themselves are very expensive and are limited to people who can come to the
observatory to participate in an activity. On the positive side, all of these models avoid the scale-related
misconceptions created by the traditional mechanical orreries.
Three Portable Orrery Models
We have created three different models that can be used in different venues, depending on the time and space
available (See Fig. 3).
 The largest, and heaviest uses aluminum pie pans (See Fig. 3 and Fig. 7, page 11). This can take as
long as 45 minutes to set up, similar to the one in the YouTube video. We have used this only when
we have extra time such as an overnight with lots of help. However, this model could also be the
basis for a more permanent model on a playground, for example. The scale for this model is 1 m =
100,000,000 km, so the Earth is 1.5 m from the Sun, with Jupiter and Saturn at about 9.6 and 14.4 m,
respectively. See Table 3 on page 14 for the other planets distances. This model requires a space of at
least 20 feet by 20 feet to accommodate the inner planets, out to Mars, with room for the
constellations. In addition, about 50 feet of space is needed to accommodate Jupiter and Saturn at
their actual model distances. Ideally this model should be constructed outside.
 The one described below (Fig. 3, center, and Fig. 4, page 3) is the one we use most often. While it
takes longer for initial construction (about a day), it is fairly light, can be carried on a plane, and can
be set up in five to ten minutes. After several trials, we now use rip stop nylon. We use this in most of
our workshops where we are limited to a typical one hour presentation. The scale of this model is 1 m
= 1 AU. 1 AU, an Astronomical Unit, is the mean distance from the Sun to the Earth, 150,000,000
km. Jupiter and Saturn are 5.2 and 9.6 meters away, respectively.
 The Tabletop Orrery is used when space is very limited. It can be set up in just a few minutes. It is
small and very portable and can be used where there may be large numbers of people who are able to
spend only a short amount of time with your activity. It major disadvantage is that participants can
only view the orrery from the outside and cannot “be” the various planets. The scale for this model is
10 cm = 1 AU (See Fig. 8).
Pie Pan Model
Detail, Portable Model in Felt
Table Top Model
Figure 3: All three versions of the floor and Tabletop orrery models. Left: the aluminum plate model,
center: an early version of the rip stop nylon model (originally felt), and right: the Tabletop model.
Human-Powered Orrery 4-15-15.docx
2
Key Concepts
With the Human-Powered Orrery, our primary goal is to connect the activity with actual observations of the
night sky. Learners, both students and educators (“students”), are able to use the positions of the planets as
modeled by the orrery to predict what they will see that evening in the night sky.










Orbits of the planets: prior knowledge (Activity 1)
Scale of the Solar System (Activity 1)
Rotation and day and night (Activity 1)
Relative positions of the planets in the night sky (Activity 1)
Predicting the night sky (Activity 1)
Revolution of the Earth and seasonal constellations (Activity 2)
Revolution and motion of the planets in the sky and relative motions of the planets in their orbits
(Activity 3)
Phases of the Moon and Venus (advanced)
Length of day vs. period of rotation (advanced)
Retrograde motion (advanced)
Setting up the Ripstop Nylon Orrery
The orrery should be in one piece (three pieces taped together) and should look like Figure 4 below:
Figure 4: Positions of the planets during a teacher workshop in July, 2011
These pictures were taken at one of our workshops. In the left image, a teacher (white blouse representing
the Earth) is positioned at sunset and can see Mercury, but Venus, Mars, and Jupiter (hidden behind “Earth”
and the author) are morning objects, so she would have to rotate 180 degrees counterclockwise (12 hours) in
The circles are numbered 1, 2, 3, etc. in a counterclockwise direction to denote the various positions of the
planets Mercury, Venus, Earth, and Mars. The separations of the circles are related to the counterclockwise
motions each of the planets around the Sun. For Venus, Earth, and Mars, each circle represents 16 days of
orbital motion. Because Mercury moves much faster in its orbit, the circles are separated by 8 day intervals.
Use Table 1 below to find where a planet is located on any given date. We use six aluminum plates—three
for Jupiter and three for Saturn. These are placed at about the proper distance and angle relative to the rest of
the orrery. If space is limited, place the plates as far away as possible and note to the students that they
should really be farther away.
Human-Powered Orrery 4-15-15.docx
3
Table 1: Orrery Planet Positions for 2015 to 2016
Date
Mar. 31, 2015
Apr. 16, 2015
May 2, 2015
May 18, 2015
June 3, 2015
June 19, 2015
July 5, 2015
July 21, 2015
Aug. 6, 2015
Aug, 22, 2015
Sept. 7, 2015
Sept. 23, 2015
Oct. 9, 2015
Oct. 25, 2015
Nov. 10, 2015
Nov. 26, 2015
Dec. 12, 2015
Dec. 28, 2015
Jan. 13, 2016
Jan. 29, 2016
Feb. 14, 2016
Mar. 1, 2016
Mar. 17, 2016
Apr. 2, 2016
Apr. 18, 2016
May 4, 2016
May 20, 2016
June 5, 2016
June 21, 2016
July 7, 2016
July 23, 2016
Aug. 8, 2016
Aug. 24, 2016
Sept. 9, 2016
Mercury
1
3
5
7
9
11
2
4
6
8
10
1
3
5
7
9
11
2
4
6
8
10
1
3
5
7
9
11
2
4
6
8
10
1
Venus
4
5
6
7
8
9
10
11
12
13
14
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1
2
3
4
5
6
7
8
9
Earth
13
14
15
16
17
18
19
20
21
22
23
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
24
Mars
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Jupiter
Saturn
4
3
5
6
4
7
8
5
9
10
6
Note: When doing this activity, we suggest that you start by “rounding down” the date, e.g., if you do the
activity on, say April 24, 2015, you should have the students first stand on circles for April 16, 2015, and
then move to the next date, May 2, to see how the positions have changed. For the following activities, we
have chosen April 16, 2015 as an example of when a presentation might be held. It is also important that all
students participate. If you are doing only one or two activities, have all of the students take turns being the
Earth and seeing what the “sky” looks like from the Earth, for example. Planets Uranus and Neptune and the
dwarf planet Pluto move more slowly through the sky and are in Pisces, Aquarius, and Sagittarius,
respectively, for the entire time. On page 15 (Fig. 9), there is a 1/30 scale version of the orrery. It is actually
a 1/3 scale model of our Tabletop Orrery and includes the positions of the constellations as well as partial
orbits for Jupiter and Saturn at 80-day and 160-day intervals, respectively. This figure can be used to
estimate the positions of the constellations around the orrery as well as the positions of Jupiter and Saturn.
Human-Powered Orrery 4-15-15.docx
4
Activities:
Below are a set of activities that can be done with the orrery. The activities are written up for either the larger
aluminum pie pan orrery (See Fig. 7) or for the smaller ripstop nylon orrery (as illustrated in Fig. 3) with the
constellations on poster board or foam board that can be put around the inner planets. If you have created the
larger version using the aluminum pans or a more permanent one, the only difference will be the size of the
Sun and the distances from the Sun to the planets.
Activity 1
Orbits of the planets: prior knowledge
Ask the students what is at the center of our Solar System. Pick one of the students who gives the correct
answer (the Sun) and have them stand at the center of the orrery. Then, ask the students which planet is
closest to the Sun. Pick one of the students who gives the correct answer (Mercury) and have them stand on
the appropriate circle (in our example, since we are doing this on April 24, 2015, they should stand on circle
3 for Mercury, “rounding down” to April 16). Do the same for Venus (circle 5), Earth (circle 14), and Mars
(circle 5). At this point, you can also do the two brightest “outer” planets, Jupiter and Saturn. Because they
are too far away from the Sun, we have only given the directions towards these planets and their distances. If
the distances are too great to allow the students to stand that far from the “Sun,” have them stand as far away
as they can. Caution: Make sure that the students representing Jupiter and Saturn are able to hear everything
that is happening so that they do lose out on the presentation or feel left out. Explain to the girls that what
they are standing on is a scale model of the Solar System. Ask them what is modeled correctly and what is
not. The relative distances from the Sun are correct as are the relative positions of the planets. However, in
this model, the Sun would be represented by a 14 mm (about 1/2 inch; 9 mm for the small model) bead and
even Jupiter would be less than a millimeter in diameter! Also, because of the way the model is set up,
Jupiter (7.8 or 5.2 meters) and Saturn 14.4 or 9.6 meters) may be “beyond” the constellations. In reality, the
nearest stars (other than the Sun) are hundreds of thousands of times farther away from the Earth than the
Sun is! At this point, you can also discuss the actual distances of the planets from the Sun and introduce the
term Astronomical Unit.
Day and night—rotation and revolution
Have the student who represents the Earth face directly toward the student who represents the Sun. Explain
to them that their nose represents a mountain on the Earth pointing directly up at the sky. Ask the first
student what the time of day is for “Mount Nose,” noon. Have them rotate counterclockwise (you should
explain or ask what this motion is) and stop them when they have made a 1/4 rotation. Again ask them what
time it is (around sunset). Have them continue their rotation through midnight, sunrise, and back to noon.
You should then note that one rotation of the Earth on its axis is equal to one day (day/night cycle) of the
Earth. At this point, you can either mention that all of the planets rotate and have day/night cycles or ask the
students what they think. You may also mention that you will be talking about orbiting (revolving) in a few
minutes. Note: At this point or prior to Activity 1, you may want to demonstrate the rotation and revolution
of the Moon. Note: the actual rotations of some of the planets, including the Earth, and how this relates to
their actual day/night cycles is somewhat more complicated and can be discussed with older students (see the
advanced activities).
The positions of the planets in the night sky
Now, have the student who represents the Earth start rotating again. As they do, have them look at the
locations of the five other planets and call them out as they rotate. Have them stop when they are facing each
planet and ask them whether or not they think that they would be able to see the planet in the real sky. What
you want to get across is that, if the Sun is up, they will not be able to see the planet. They will be able to see
any given planet only if the Sun has set and it is dark! This is a good point at which to get other students to
take the role of the Earth and to look at the other planets in the sky.
Human-Powered Orrery 4-15-15.docx
5
Predicting the night sky
Have students who are not one of the planets or the Sun stand by the constellations that are “behind” the
planets and have them call out the name of their constellation one at a time starting from the direction of the
lowest numbered circles (Pisces), holding their constellations, if they have them. Note that when they go
outside, they should be able to see this planet “in” this constellation (if it is dark). Now, ask all of the
students to predict what they would expect to see if they went out after sunset: what planets would be in the
night sky, where would they be or at what time would they have to go out to observe them, and what
constellations would they see. If this activity is followed up by an evening event, they will be able to follow
up their predictions with actual observations!
Activity 2
Revolution of the Earth and the seasonal constellations
For this activity, you should just have the Earth orbit (revolve) around the Sun. To engage the most students,
have a student stand on each of the constellation positions. Use the constellation stick figures if you have
these. First, have the student who represents the Earth stand looking in the direction of the Sun (something
you should not do with the real Sun). Ask them what constellations are behind the Sun and if they would
expect to see these constellations in the real sky (no, they are “up” during the day and “behind” the bright
Sun). Have the student rotate 1/4 turn counterclockwise and ask them what constellations they see and then
have them move an additional 1/4 turn (midnight) and ask them what constellations they see. This can be
repeated for sunrise. Have the student now move from one circle to the next (circle 15, May 2), one step in a
counterclockwise direction. Explain to the students (or ask them) that this represents the motion of the Earth
in its orbit around the Sun (orbiting, or revolving). Ask the students what the Earth is doing as it moves from
one circle to the next (it rotates) and mention that, since each step represents 16 days, the Earth rotates 16
times (if the student wants to rotate 16 times, have them do it slowly!). As time allows, have the Earth step
either 5 additional steps (1/4 of the way around the Sun, circle 20, July 21) or 11 additional steps (1/2 of the
way around the Sun, circle 3, October 25). Ask the students how much time has passed (about three or six
months for 5 and 11 steps, respectively). Ask the Earth to again rotate and describe what constellations they
see (the constellations that were “up” at night cannot be seen because they are “behind” the Sun). This
describes why we see different constellations at different times of the year.
Activity 3
Revolution—motion of the planets
As time permits, you can now talk about the revolution (orbit) of all of the planets around the Sun. This is a
more advanced activity and best done with upper elementary or middle school students. Have “Earth” move
back to its original position (circle 14). Explain to the students that you will now have all the planets orbit
(revolve around) the Sun. Have the students who represent each of the planets move from one circle to the
next. In the case of Mercury, this should be two steps, noting that Mercury moves faster in its orbit. At this
point, there are two concepts that are being addressed: the motion of the planets in the night sky and the
orbital motion of the planets. Note: At some point, the students may ask about the rotation of the other
planets. On Page 8, we have provided you with additional information about the planets, including their
period of rotation.
Motion of the planets in the night sky
Now that the students (except for Jupiter and Saturn, who take one step only after everyone else has taken
five steps for Jupiter and 10 steps for Saturn) have taken one step, have the student who represents the Earth
look at the other “planets” and “constellations.” Have them rotate, saying the name of each planet as they see
them and the constellations they are “in.” Ask the students if the planets are in the same positions relative to
the Sun and constellations after 16 days. Have the “planets” take another step and repeat. The students
should notice that, as time goes on, the planets are moving relative to each other, relative to the
Human-Powered Orrery 4-15-15.docx
6
constellations, and relative to the Sun (some may disappear in the glow of the evening Sun while others may
appear in the morning sky). Have the students make predictions about the movement of the planets over the
next month or two and write this down. Use it the next time you meet with the students at night and see if
their predictions were correct.
Relative motions of the planets in their orbits
Now that the students who represent the six planets have been moving around the Sun, ask them if they have
noticed anything about the motions of the planets. For example, Mercury takes less time to go around the
Sun because its orbit is shorter, but at the same time, it is moving faster than the other planets; Jupiter and
Saturn, with much bigger orbits, are moving much more slowly in their orbits since the plates are separated
by 160-day intervals.
Advanced Activities
Phases of Venus
Note: This activity may require some prior knowledge—the cause of the Moon’s phases. This should be
done with only Earth and Venus. Have Earth stand on circle 14 and Venus on circle 5 (April 16, 2015). Have
Venus put their arms out straight from their sides while facing the Sun. Ask the students what the area facing
the Sun represents (daytime on Venus). Note to the students that they will be modeling the part of Venus that
is in daylight and not worry about the fact that Venus is rotating. Ask the students if they would expect to see
Venus in the sky (yes, in the evening sky) and what part of Venus do they “see” (about half of the
illuminated side, “a gibbous Venus”). Have the two students move from circle to circle, such that Venus is
always facing the Sun (remind them that all the while Venus is rotating so that the same side of Venus is not
always facing the Sun). As Venus and Earth orbit around the Sun, ask the students two things: what time of
day or night would we see Venus (in the evening) and how much of Venus’ day side can Earth see (less of it
over time). They are modeling the phases of Venus! They may also notice that when Venus was fuller, it was
farther from the Earth (smaller) and as it goes through its phases, it gets closer and bigger. It will be biggest
when it is “new” (we “see” the unlit night side of Venus). This will happen in August of 2015.
Length of day vs. period of rotation
Note: Over the years, we have seen many K–12 astronomy books gloss over this topic and either ignore it or
give vague answers. The following activity is not that difficult and we have done it many times with middle
school children and, at a basic level, they understand it. We will demonstrate this for the Earth as it is harder
to do for Mercury and Venus. There are two ways to measure time: sidereal (relative to the stars) time
measures how long it takes a planet, such as the Earth, to rotate once (which we demonstrated above). We
can also measure time by the Sun, say noon to noon, which we call synodic (relative to the Sun) time, the
length of a day (day/night cycle).
As we have already demonstrated with the seasonal constellations, as the Earth orbits the Sun, we see
different constellations in the night sky. If you were to go out each night, you would notice that the stars
appear to rise (the Earth’s rotation causes the objects in the sky to appear to rise in the east, move across the
sky, and set in the west) about four minutes earlier each night. Why is that? For this activity, you will need
only two students—the Sun and the Earth. Have the Sun stand in the center of the orrery and have the student
who represents the Earth stand facing the Sun with one of the walls behind the Sun (Fig. 4). Because we are
using real positions of the Earth, we suggest that the student start off on circle 1. Ask them (or all of the
students) what time of day it is (“Mount Nose” facing the Sun = noon). Now, have them move from circle 1
to circle 2. While doing this they should try to rotate 16 times so that they end up on circle 2 and back to
facing the wall. Ask them (or all of the students) what the time of day is. They should notice that it is not
Human-Powered Orrery 4-15-15.docx
7
Figure 5: From circle 1 to circle 2:
16 rotations of the Earth and 16 days
quite noon and that for it to be noon, the student needs to rotate a
little more (about 16 degrees). At this point you can just state that,
in 16 days, because the Earth has been rotating on its axis and
revolving around the Sun, the Earth has to rotate about 16 degrees
more for “Mount Nose” to be facing the Sun and for it to be noon.
This means that each day, once the stars (if you could see them)
are in the same place in the sky, the Earth has to rotate 1 extra
degree for the Sun to be in the same place in the sky, which is
almost four minutes. However, since we measure time by the Sun
(one day equals 24 hours exactly), the stars appear to rise four
minutes earlier each day. Therefore, while our day is 24.0 hours
long, the Earth actually rotates once in 23 hours 56 minutes and 4
seconds In 365 synodic (relative to the Sun) days, this adds up to
one extra rotation of the Earth—in one year there are 366 sidereal
(relative to the stars) days. The stars and constellations are back
where they were one year before relative to the Sun!
As you can see from Table 2 below, for the outer planets, where they are moving more slowly, there is virtually no
difference between rotation time and the length of their days. However, for Mercury and Venus, it is much more
complicated because their rotation rates are large compared to the orbital rates. Also, Venus rotates opposite to its
direction of revolution.
Table 2: Basic information on the Sun and planets
Sun
Distance
Length
Period
Length
Diameter
Diameter
Planet
From Sun
of
of
of
(kilometers)
(scale model)
(AU)
Year
Rotation
Day
Large (Small)
Sun
0.00
N/A
24.47 days
N/A
1,392,000
13.9 (9.30) mm
Mercury 0.38
87.97 days
58.65 days
175.94 days
4,880
0.05 (0.03) mm
Venus
0.72
224.7 days
-248.02 days
116.75 days
12,104
0.12 (0.09) mm
Earth
1.00
365.26 days
23.93 hours
24.00 hours
12,756
0.13 (0.09) mm
Mars
1.52
1.88 years
24.62 hours
24.66 hours
6,792
0.07 (0.05) mm
Jupiter
5.20
11.86 years
9.92 hours
9.92 hours
142,980
1.4 (1.0) mm
Saturn
9.58
29.46 years
10.54 hours
10.54 hours
120,540
1.2 (0.8) mm
Uranus
19.23
84.32 years
-17.24 hours
17.24 hours
51,120
0.5 (0.3) mm
Neptune
30.10
464.79 years
16.11 hours
16.11 hours
49,530
0.5 (0.3) mm
Pluto
39.26
248.09 years
-6.39 days
6.39 days
2,306 0.023 (0.015) mm
Retrograde Motion
As can be seen in Figure 9 on Page 15, the constellations on the Tabletop Orrery lie just outside the orbit of
Mars. Even with the Human Orrery, because of space limitations, the constellations are generally closer to
the Sun than either Jupiter or Saturn. If time allows, their true distances and their 3D nature can be discussed
during the activities. There is a more detailed discussion of this in the next section. However, if you can put
the constellations fairly far from the orrery itself, you have the opportunity to model retrograde motion—the
apparent loops, relative to the distant stars, that the planets beyond the Earth make as the Earth moves past
them (opposition) in its orbit (Fig. 5). For Earth and Mars this occurred in March 2012. It happened again in
April of 2014 when Mars was “in” Virgo. Mars will be in retrograde again in May 2016. Have three students
represent Libra, Scorpius, and Ophiuchus as far away from the orrery as possible. Have two students be
Earth and Mars and have them step off several months from March 17, 2016 (circles 12 and 26) through July
23 (circles 20 and 34) and have them describe what they see. Have other students model this so that as many
as possible can participate. This apparent backwards motion of Mars is similar to what happens when you
Human-Powered Orrery 4-15-15.docx
8
pass another car going along a highway. If you are going 65 miles per hour and, as you pass, you look out the
window at a car in the next lane going 55 miles per hour, it will look as if the other car is going backwards.
Figure 6: Mars in the Loop, C. E. Tenzel and T.
Tezel (The World at Night), 2012; published
Astronomy Picture of the Day Site, 2012 (APOD,
authors and editors: R. Nemiroff [MTU] and J.
Bonnell [UMCP])
Additional Activities: To the Solar System and Beyond
With spacecraft in orbit around other planets, missions to two dwarf planets, spacecraft that are “just leaving
the Solar System,” and spacecraft and ground-based telescopes discovering hundreds of planets around other
stars, there are many other opportunities to use the Human Orrery. Below are several ideas for activities that
you might consider and links to relevant mission sites:
1. Model the path of the Dawn spacecraft from its launch from Earth in 2007 to its arrival at Vesta in
2011. Then model its continuing journey from Vesta in 2012 to its arrival at the dwarf planet Ceres in
2015 (please contact the author for the orrery positions for Vesta and Ceres). The path of the Dawn
spacecraft can be found at: http://dawn.jpl.nasa.gov/ and
http://dawn.jpl.nasa.gov/mission/trajectory.asp.
2. Model the path of the New Horizons spacecraft that will fly by the dwarf planet Pluto in 2015. The
path of the New Horizons spacecraft can be found at: http://pluto.jhuapl.edu/mission/whereis_nh.php.
3. Voyager 1 will be the first spacecraft to leave the Solar System (the heliosphere). You can plot its
path and see where it is relative to the Kuiper belt (more than twice the distance of its outer edge) and
to the Oort cloud (probably not even 10 percent of the distance to the inner edge of the inner Oort
cloud). The path of the Voyager spacecraft can be found at: http://voyager.jpl.nasa.gov/
4. The Voyager spacecraft were not the first spacecraft to travel beyond the planets. Pioneers 10 and 11,
now silent, are still moving away from the Sun:
http://www.nasa.gov/centers/ames/missions/archive/pioneer.html,
http://en.wikipedia.org/wiki/Pioneer_10, http://en.wikipedia.org/wiki/Pioneer_11, and
http://azstarnet.com/news/science/scientist-feels-tie-to-distant-spacecraft/article_e3d74856-29665e63-b5bb-5098b2501caa.html
5. Create other planetary systems using the orrery and calculate their relative distances from our Solar
System. Planetary systems have been discovered with ground-based telescopes and orbiting
telescopes. Methods of discovery include direct imaging, transits, and radial velocity (Doppler). Each
one of these is biased toward distance from their host star, the planes of their orbits, or size.
Information about extra-solar planets can be found at a number of sites: http://kepler.nasa.gov/,
http://www.jwst.nasa.gov/origins.html, and http://www.eso.org/public/videos/eso1035g/.
Human-Powered Orrery 4-15-15.docx
9
Lessons Learned and Model Limitations
Engaging students
What is critical with any model is to engage the participants—if at all possible everyone should participate.
A big part of any modeling needs to be a discussion about the types of models (mechanical, mathematical,
computer-generated, etc.) and their limitations. This helps to identify and rectify misconceptions and helps to
avoid creating new ones.
Model limitations
As with any model, there are some things that we are modeling accurately, such as the scale of the distances
of the planets from the Sun, and other things that we are not, such as the elliptical orbits of the planet. Model
limitations should be discussed and be prepared to answer some other questions. The two questions that
generally come up are: 1) how “off” are the planets due to the assumption of circular orbits (generally about
the size of one of the circles or less) and 2), since it is assumed that the planet years are an even multiple of
16 days (8 for Mercury), how long is it before you have to correct for this (4 or 5 years/orbits for Earth and
less often for the other planets). Another question that may come up is why the Sun’s position is not the same
as the zodiacal Sun sign (precession).
Another modeling inaccuracy comes from the placing of the constellations. As we mentioned above, the
distances to the stars and constellations cannot be modeled accurately. Even with the Human Orrery, because
of space limitations, the constellations are generally closer to the Sun than either Jupiter or Saturn. This can
lead to further discussions of their true distances. The stars in the constellations we use are generally from a
few tens of light-years (the distance light travels in a year, 63,241 AU) to hundreds of light-years away and
some are over a thousand light-years away. On the scale of the orrery, this implies that the stars in our
constellations are thousands of kilometers away! Because the constellations are not far off relative to their
true distances, the planets may be as much as a whole constellation off (viewed on the orrery vs. the true
sky).
Model limitation, Kepler’s Third Law
The planets may also be “off” in their orbital positions because of Kepler’s Third Law. An individual planet
in its elliptical orbit moves faster when it is closer to the Sun and slower when it is father away from the Sun.
Since the orrery uses circular orbits and mean motions, sometimes the orrery position will be ahead or
behind the true position. This is most evident with Mars in its orbit and was one of the great mysteries of
astronomy that could not be resolved until Kepler developed his Laws!
Connecting the Orrery to the Next Generation Standards:
The Next Generation Science Standards (NGSS, Achieve, Inc., 2013) was released in June 2013. While
specific mention of constellations was eliminated in the final version, the Human Orrery still aligns well with
the NGSS Performance Expectations.
In the Fifth Grade Storyline, “Students are expected to develop an understanding of patterns of…day and
night, and the seasonal appearance of some stars in the night sky. The crosscutting concepts of patterns;
cause and effect; scale, proportion…and systems and systems models are called out as organizing concepts
for these disciplinary core ideas. In the fifth grade performance expectations, students are expected to
demonstrate grade-appropriate proficiency in developing and using models, planning and carrying out
investigations, analyzing and interpreting data…evaluating, and communicating information; and to use
these practices to demonstrate understanding of the core ideas.” While the students are no longer expected to
identify constellations (eliminated after the final public review), one could argue that students would not be
able to identify any seasonal star patterns without the aid of constellation patterns!
Human-Powered Orrery 4-15-15.docx
10
In the Middle-School Timeline, “students can examine the Earth’s place in relation to the solar system,
Milky Way galaxy, and universe. There is a strong emphasis on a systems approach, using models of the
solar system to explain astronomical and other observations of the cyclic patterns of eclipses, tides, and
seasons.” Again, the Human Orrery is an appropriate tool for connecting the students to the real motions of
the Earth and the planets of the Solar System.
Constructing a Human Orrery
The following directions are meant only as guidelines for the construction of the orrery. We have tried
several different methods, some more elaborate than others. Ideally, what you want to end up with is a very
small circle at the center to represent the Sun with four rings of plates or circles representing the orbital
positions at 16-day intervals of the four inner, terrestrial planets: Mercury, Venus, Earth, and Mars. If there is
enough room, you may also want to include partial orbital positions for Jupiter (80 day intervals), and Saturn
(160 day intervals) as they are farther from the Sun and move much more slowly around the Sun. We will
first describe the methods for setup and then give the details for how to position the circles that represent the
positions of the planets in their orbit around the Sun.
The Original Portable Orrery (Large Model):
This orrery is made with 6-inch aluminum pizza pie plates. This was first constructed with the help of a
group of girls from Amphitheater Middle School (Fig. 7 on the left). They helped number the plates and
decorate some of the plates with the symbols for the planets. On the right is a picture of the materials used to
create the model. The lengths of the strings represent the scale model distances of the planets from the Sun
and we used green plant stakes for marking the separation of the plates on the ground. The strings are hooked
to the metal rod with shower curtain hooks. Two people could set this up in under 30 minutes: one person
moves around the central rod while the other places the pie plates down at the proper intervals. As an
alternate, as shown in the materials list below, you can just use one string to draw a circle (for each orbit) for
the pie plate placement. This is the method that we will describe below.
Figure 7: Girl Scouts from Amphitheater Middle School built an orrery using aluminum pie pans (left). The girls are
shown laying out the pie pans, some of which they personally painted (center). The image on the right shows the
method they used for placing the plates using two strings tied to a green plant stake. The description below can be used
to recreate the model pictured to the left, above. You can use aluminum pie pans, as we did, or draw circles on the
ground (which can be made more permanent).
Materials
6 stakes that can be stuck in the ground or held in place by a person
6 strong strings (lengths described below)
1 meter stick
A template for drawing circles (6 inches is ideal; we used a sturdy paper plate with a hole in the center so
that the position marks could be seen)
Human-Powered Orrery 4-15-15.docx
11
Chalk or some other non-permanent marker
Templates for the constellations (available from the author)
Optional:
103 6-aluminum pie plates or heavy duty paper plates (if done outside, the paper ones may blow around)
Background information
In order to do the five visible planets plus Earth (Mercury to Saturn), you will need an area about 16 meters
(55 feet) on a side. Note: if space is a problem, then you can put Jupiter and Saturn as far away as possible
and just state that they are farther away!
The easiest way we found to lay out the model is to have a stake (or pole) with a rope tied to it. For ease of
construction, we recommend four stakes and four strings with a piece of white chalk tied to the end for each
of the four inner planets. The fifth stake and string are used to make the centerline (“up” from the Sun) from
which we do the measurement. The sixth stake and string are used for both Jupiter and Saturn. When doing
this on your own or in a place where you cannot draw orbit circles (as was the case with the Amphitheater
girls), the method shown in Figure 7 (right) works well. But drawing circles for the orbits of the planets
before marking down the plate positions was easier.
The scale of this model is 1 meter equals 100,000,000 kilometers. Therefore, you will need strings, that,
when tied to the stake on one end and with the chalk tied at the other end will be able to make circles that are
equal to the mean scale distances of the planets.
Note: the numbers given below are accurate through at least the end of 2016. We discuss the accuracy of this
model below.
Procedure
1. Lay down the centerline string. It can be either laid on the ground or, if possible, staked down and
stretched out from the Sun position. It needs to be as long as the distance to the farthest planet that you
will have on your orrery, 2.6 meters if just going out to Mars or 15 meters if going all the way out to
Saturn.
2. Starting with Mercury, one person should hold the stake at the center of the orrery (the Sun position).
The other person will draw an orbit circle that will represent the orbit of Mercury. Make sure that the
stake is held securely and the string is taught. It is not critical to make a perfect circle; being out of round
by a centimeter or two will not be very noticeable in the final product.
3. Continue doing this for Venus, Earth, and Mars.
4. Using the long (Jupiter/Saturn) string, draw circles that represent the orbits of Jupiter and Saturn. Since
these planets move more slowly than the inner planets, you may choose to draw only the part of the
orbits that Jupiter and Saturn will be in over the next few years. Again, make sure that the stake is secure
and that the string is taught.
5. Next, using Table 3 (page 14), put an “X” on the appropriate orbit circle to indicate the position of the
first plate for each planet. Using the meter stick and the spacing between each position of the planets, put
an “X” for the second position. Continue this until you have marked all of the positions around the circle.
Note: nobody is perfect! The spacing between the last position and the first position may not come out
exactly even. If you are off a little you can just adjust the spacing of all or some of the plate positions
(see the discussion of accuracy below). Once you are happy with your positions, then, use the template to
draw all of the circles. Note that for Jupiter and Saturn, you will just need to estimate the initial positions
based on the orrery on Page 12.
6. Note on planets, etc.: Because Uranus, Neptune, and Pluto (some of us still consider Pluto to be a
planet) move more slowly through the constellations, we have just given the constellations they are in for
the two years covered by the tables on page 4. The first four asteroids and the dwarf planets (Ceres is
Human-Powered Orrery 4-15-15.docx
12
considered to be both) are too faint to be seen without a telescope. If you are interested in their positions,
please contact Larry Lebofsky (email on page 1).
7. A note on accuracy: The orbits that are created here are circular. The true elliptical orbits can be done,
but are beyond the scope of what we are trying to do here. If you want to do accurate elliptical orbits, see:
http://star.arm.ac.uk/orrery/make_orrery_2011jul04.pdf. Therefore the positions that we create with this
model may be off in distance from the Sun (due to the shape of the orbits) and off along the orbit (by a
few centimeters) due to the slightly changing velocity of the true planets in their orbits. Also, if you use
this model for several years, you might need to skip a plate as you walk around to align positions with
dates. This is because, except for, Mercury, the 16-day intervals do not give you an exact number of days
in the planets’ year. For example, with 23 plates, Earth’s “year” is 368 days, not 365. So, after five years,
your position on the model will be off by a whole plate relative to the constellations.
The Ripstop Nylon Orrery (Smaller Model):
In the past, we used felt and vinyl. Felt got dirty quickly and vinyl was bulky and heavy. Our present
portable orrery (small model) uses ripstop nylon. This is fairly light and fits in a small suitcase for transport.
The method of construction in the previous section can also be used to create this model. However, you will
probably not want to put a hole in the center of the cloth, so you will want to use something like a wooden
dowel to secure the end of the strings. We had felt that was 6 feet wide (Fig. 3) which was sufficient for the
width of the model (12 feet, with room to show the directions to Jupiter and Saturn). The ripstop nylon is
about 5 feet wide, so requires three strips. For ease of construction and transport, we used duct tape (we
found colored tape) to tape the strips together before construction. One person holds the dowel while the
other draws the orbits for the planets. For a model on the ground, as above, you will need a space at least 20
feet by 20 feet. For the ripstop nylon model, you will need a space at least 15 feet by 15 feet. Since the
material comes in strips, it is easy to draw a centerline with the chalk. Also, since we do only partial orbits
for Jupiter and Saturn, we just have the students who are Jupiter and Saturn stand at the proper distance and
position relative to the inner planets (Fig. 9).
Materials
1 to 5 dowels that can be held in place by a person
4 strong strings (lengths described below)
1 meter stick
A template for drawing circles (6 inches is ideal; we used a sturdy paper plate with a hole in the center so
that the position marks could be seen)
Chalk (sidewalk or similar chalk as it is softer than chalkboard chalk) or some other non-permanent marker
Permanent marker for drawing the circles*
Templates for the constellations (available from the author)
The Tabletop Orrery:
This model exists as a PDF file that is available from the author. It is 21 inches wide and can be printed out
on a large format printer or at a copy store as a small poster. We use small wooden figures (Meeples;
www.meeplesource.com) but Lego® figures can also be used (See Fig. 8).
*Caution! A group that we work with who were the first to use ripstop nylon neglected to realize that the
nylon, if it has no backing, is porous. Because of this, they now have a carpet with circles on it. Please make
sure that you make the model on a hard, protected surface!
Human-Powered Orrery 4-15-15.docx
13
Table 3: Data for Constructing the Orrery, Large (Small)
Object
Centerline
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Sun Dist.,
AU*
0.39
0.72
1.00
1.52
5.20
9.58
19.19
30.07
39.48
String Length,
meters*
Day
Interval
2.5-15 (N/A)
0.58 (0.39)
1.08 (0.72)
1.50 (1.00)
2.28 (1.52)
7.80 (5.20)
14.37 (9.58)
28.71 (N/A)
44.98 (N/A)
59.06 (N/A)
8
16
16
16
80
160
N/A
N/A
N/A
Number of
planet circles
11
14
23
43
16
8
N/A
N/A
N/A
Location, Circle #1
(Centimeters Left of
Centerline)
Planet Circle
Spacing
(Centimeters)
19 (13)
21 (14)
4 (3)
9 (6)
N/A
N/A
“In” Pisces
“In” Aquarius
“In” Sagittarius
33.1 (22.3)
48.4 (32.3)
41.0 (27.3)
33.3 (22.2)
90.8 (60.5)
134.7 (89.8)
N/A
N/A
N/A
*For the large model the scale is 1 m = 100,000,000 km;
for the ripstop nylon model, the scale is 1 m = 150,000,000 km (1 Astronomical Unit)
Tabletop Orrery
Figure 8: Tabletop Orrery with Meeple characters from above (left) and viewing from behind Earth at dawn
(orange Sun) viewing Venus (to the right of the Sun) and Saturn (far right). (Credit: Larry Lebofsky)
Human-Powered Orrery 4-15-15.docx
14
Figure 9: Table Top Orrery rescaled to fit on this page (one-third scale of the Table Top Model, 1/30 scale of the
small orrery). Colored circles are for March 31, April 16, and May 2, 2015.
Human-Powered Orrery 4-15-15.docx
15