We can test how our night perception can be improved with dark adaption by recording the faintest star we can see after various periods of dark adaptation. A more accurate method is to count the number of stars visible in a well defined area of sky however, this is difficult for novices.

We start by identifying the stars that will be visible when we step outside. This requires a dry run and use of the star map. There are several bright asterisms marked on the map with thick grey lines. We can start with these. Using the date scale along the bottom of the map and the time of the experiment, determine which asterism will be visible outside the door. Verify this by stepping outside and identifying the asterism.

Stepping back indoors, notice the various size of star-dots in the area and their relative position to the asterism. These will be used to estimate the sensitivity of your eyes. The smaller dots are fainter than the larger ones. The map is designed to show the stars visible from a city. Under a dark rural sky, you will see many more stars than are shown on the star maps. So many in fact that, recognizing the constellations is made more difficult by the large number of fainter stars.

Compare the size of the stars you labelled with the brightness scale (magnitude) to the left of the round Polar Chart. You can estimate the visual limit (limiting magnitude) for your sky for various degrees of dark adaptation.

If you can see stars that are fainter than those plotted on the map - Congradulations, you have a very dark sky for an urban site. You may also be able to see some of the celestial objects that are plotted on your map.

If you have the Starlight Theatre's laminated star maps, you will be able to read the limiting magnitudes directly off your map because a sequence of stars are labelled with standard magnitudes.

In summary, you will experience how much you vision improves with dark adaptation. It is remarkable how sensitive your eyes can become, if you give them a chance.

These satellites preform various functions but most are Earth observation satellites carrying "remote sensing" payloads. Their low orbits cause them to travel relatively quickly across our sky compared to the stars. They may take 10-minutes to travel from horizon t horizon. Most of these orbit approaximately north to south (or south to north). They pass about 10 degrees to the east or west of the North Star and are called "polar orbiting" satellites.

They can have some interesting characteristics. They may fade and grown brighter as they rotate or change they way they reflect the sunlight. They may also fade from view before they reach the horizon, or they may become visible well up in the sky. This is because they will pass into and out of the Earth's shadow as they orbit the Earth. There are more extensive articles about artifical satellites. You may visit (this site or this one) for example.

This project asks that you plot the path of a satellite on your star map. Either stay out for about 2-hours or return after about 95 minutes from the observation of the first satellite, and plot its second path acros the sky. If you stay out for the full 2-hours, you will observe the passage of several statellites in thsi time period.

From the time of its passage, their orbital period and the closest distance they pass by the North Celestial Pole, you can identify which satellite you observed. There may be several candidates, but this does not weaken the excitment of observing these objects.

A word of caution is in order. Although ther are a lot of satellites visible on any clear night, ther eare a LOT MORE air planes. You may have to watch th eobject very carefully to ensure that it is not an air plane. Planes may change course, they have navigational lightings that may flash (though high altitude planes may be too far away to see their flashing lights.

We remember warm summers with long days and the Sun high in the sky at noon. Contrast this with winter with shorter days and a low sun at noon. There is a slow progression from one to the other that is more easily understond through observations of the setting (or rising) Sun and our starmap.

We will first describe how to make the observations. Find a spot from where we will make our observations. We will be returning to this spot every few days so it must be accessible throughout winter. Plot or photograph the entire horizon from the northeast to the south and up to the northwest. We will use the features around the horizon to help us plot our observations. If you use photographs, tape them together to make a mosaic of your horizon. Then, lay a transparent plastic sheet over the mosaic to protect the images while you plot your data.

At least twice a week, plot on your mosaic, the position of the Sun at three times: 1 hour before sunset, 20 minutes before sunset and 5 minutes before sunset. Also write down the times of each observation and the time of sunset. This may also be done for the morning sky after sunrise.

If the sheet begins to look too cluttered and hard to read, trace the horizon on the plastic overlay, and replace it with a new overlay. Be sure to safely file the orginal overlay until the end of our set of observations near the end of the school year.

Continue plotting the Sun and recording the times until May or June.

What do we see in the plots and the recorded times? First, if you use "daylight saving" time, you must change the recorded times to "standard time" by subtracting one hour from the recorded daylight saving times.

You will notice the following general observations in your data: 1. Sunset gets progressively earlier from sptember until the end of December. The Sun will then begin to set earlier in the evenings after that time. 2. Each evening after September, the point of sunset will move from the western horizon towards the southern horizon. After late December, the point of sunset will move progressively northward into the northwest. 3. The length of time the Sun is above the horizon (the amount of daylight) will decrease from September to the third week in December. After that, the number of daylight hours will increase. 4. The angle that the sun sets will become shallower up to December 21. After that the angle will increase.

Straighten out the map. The stars on the celestial equator rise above th eastern horizon at the compliment angle of our latitude on Earth (90 degrees - latitude). Looking due south, the stars move horizontally with respect to our southern horizon. In the west these stars will set at the same angle with respect to the horizon. What about the stars that are south of the equator?

More southernly stars on the celestial sphere will rise above the horizon further to the south along the horizon, and will similarly set further to the south of the western horizon. Their path across the sky will be shorter than the stars on the celestial equator. Also, the stars north of the celestial equator will rise and set further to the north along the horizon and will take a longer path across the sky. This shows that the southern stars will be above the horizon for a shorter time than the more northern stars. A line drawn through the locations of the Sun before sunset, show the curve of the Sun's path as it sets. In summer the path curves up, to the north. In winter it curves down towards the south.

The ecliptic weaves above and below the celestial equator and represents the Sun's position on the celestial sphere at various times of the year. In the Summer, the Sun is high in our sky (in the constellation Gemini), so it's path across our summer sky will be long and we will have about 16 daylight hours. In winter, the Sun is low (in the constellation Sagitarius) so it will be above the horizon for a relatively short period of time.

This explains the varying length of the day from summer to winter. But, what about the temperature extremes of the seasons?

If the Sun is not above the horizon for very long, it can not heat the land as much as when it is up for a longer period of time - so the winters are cold. Also, the low Sun in winter shines its light and heat down at a low angle which reduces the heating affect. In summer, the Sunlight hits the land at an angle closer to the vertical and more affectively heats the ground.

The seasons are more easily understood with the help of the star map and by imagining the Earth as it orbits the Sun with its rotational axis (indicated by the celestial equator) tilted with respect to its orbital plan (the ecliptic).

Can we make similar observations? Not easily. Tycho did not have use of a telescope. He used carefully crafted instruments to make his very precise observations. Even without his instruments, we can time how long the Earth will take to orbit the Sun. To do this requires careful observations and the application of very important yet simple methods of observation.

The rotation of the Earth about its axis is virtually constant to within a small fraction of a second each year. We see the stars rising in the east and setting in the west. This is shown very clearly in the Celestial Sphere videotape. If the Earth did not move then every day the angle stars would rise at the same time in the night. The angle between the Sun, the earth and the star would not change. To text this, we find a convenient place to observe. Find a star that is close to the celestial equator that is near a landmark near the horizon (a tree top, a roof line or tall building). We accurately mark the spot and note the relative position of a star with respect to the landmark. Make a quick sketch in an observation journal that will help you remember the view and write down the time to within a minute.

Return to the observing point on the next few clear nights and draw the view being sure to plot the star on the drawing at precisely the same time as the first. Repeat the observations over a week. With several sets of observations in the student's journal they can test their assumption of the stationary Earth.

Remember, if the Earth does not move, the stars should rise at the same time each night. Consequently, they should appear in the same place every night. Now compare the drawings. Your observations will show that the position of the star does not appear in the same place at the same time. If it was in the east, it will appear higher in the sky. If it was in the west, it will appear lower in the sky. And, if it were in the south, it would appear farther to the right each day.

From these simple observations, we must conclude hat the Earth is not stationary. The angle between the Sun-Earth and the Star changes from night to night. To explain this, either one or all of these is moving.

This is not a simple deduction. It was not until over 200 years after Brahe and Kepler that in 1838 Friedrich Bessel was able to measure the apparent shift in the stars that result from the Earth orbit about the Sun. However, this does not render our observations mote. You may use your observations to better understand the Earth's orbit and the changing angle between the Sun, Earth and stars.

Return to your observing spot. Extend your arm, and using your index finger estimate the shift in the star's position between each observation. You can determine the angular width of your finger by measuring its width (w) and the distance between our eye and your finger with an outstretched arm (L). The ratio of these is the angular width in "radians" (about 57.3 degrees). Using this small angle approximation, the angular width of your index finger is about 1 3/4 degrees. You will find that the star will shift in its position by a bit more than 1/2 your finger's width {actually about 1 degree or more precisely 0.98 degrees).

With these numbers we discover that at 0.98 degrees per day, the sky will revolve once in 365.25 days - one year (0.98 x 365.25 = 360 degrees). If the sky appears to revolve once a year, by changing our perspective, we observe that the Earth revolves around the Sun in one year.

This one-degree shift each night means that a telescope that is designed to track the stars throughout the night must revolve a bit faster than the nominal rotation of the earth. Where the time from high noon to the next high noon is one "solar day", the motion of a telescope designed to track the stars is based on a "sidereal day" (one that is based on the stars), which is about 4 minutes faster than a solar day.

Most people only occasionally notice the Moon. Sometimes it is in the west, or east. Some times it is almost overhead and at other times it is low in the south. With the help of the star map and a regular set of observations this apparent chaotic motion is rendered into a predictable path.

The Moon's orbit around the Earth is not related to the Earth's orbit around he Sun. As the Earth orbits the Sun it defines the plane of the solar system call the ecliptic plane. Projecting this plane out onto the celestial sphere creates a line around the sky where they intersect. Viewed from the Earth, the Sun appears to drift around the celestial sphere along the line of the ecliptic. This is the wavy line across the star map.

The Moon's orbit happens to be inclined to the ecliptic plane by 5.16 degrees. Therefore over the period of the Moon's orbit (about four weeks) it will drift above and below the ecliptic but never drifting more than 5.16 degrees from the ecliptic.

A second point is that the Moon is illuminated by the Sun so, as it moves around the celestial sphere and the angle between the Sun, Moon and the Earth change, so will the illumination and the appearance of the Moon. When the Moon is in the evening sky, the Sun illuminates most of the back surface of the Moon. Only a thin sliver (or crescent) of the illuminated face is visible from the Earth. If we see the Moon in the east just after sunset, the entire illuminated face of the Moon is then visible and we see a "full moon". If we get up just before dawn and look to the east, we may see a thin crescent of the illuminated Moon. However, this crescent is reversed from the one seen in the evening sky. Here, the Sun is shining from the lower left of the Moon illuminating most of the hidden face and allowing us to only see the left hand crescent. The changing appearance of the illuminated face of the Moon is called the Moon's "phase".

The student should make a table of the date of each observation, an estimate of the area of the illuminated face of the Moon and the distance between the Sun on that date and the Moon. The location of the Sun is plotted for the first of each month. For later dates, interpolate its position with in that month.

By drawing a smooth line through the plotted points, the students will see how the Moon undulates above and below the ecliptic. In winter, when the ecliptic is high in the night sky, the Moon can appear almost overhead if it is north of the ecliptic. Whereas in the summer when the ecliptic is low in the night sky, the Moon may be very low in the sky if it is south of the ecliptic at that time. The inclination of the Moon's orbit can be measured by reading off the map the greatest distance the Moon's path extends above and below the ecliptic.

The students will also be able to relate the phase of the Moon to its distance from the Sun around the sky. The map collapses the three dimensional sky onto a two dimensional piece of paper so, there is some distortion in the geometry. However, the maps from Starlight Theatre have far less distortion than many other inexpensive maps.

The Sun is too bright to observe directly. Staring at the Sun will cause permanent blindness and should be avoided. Small telescopes and especially binoculars are very dangerous and a number of precautions must be taken for safe solar observing. Although this project does not require you to directly observe the Sun, we will mention a few safety precautions when using a telescope or binoculars.

Safe Solar Observing

Telescopes and binoculars will concentrate as much as 150 times more sunlight on the eye than unaided-eye observing. It is then focused onto your retina in your eye. This image is so bright and hot it will kill them. The intense heating may not be felt, but it will "cook" the cells causing permanent blindness. Binoculars are especially dangerous because they gather much more light than the lens in our eye and its low magnification will concentrate the light into a small image on your retina. Concentrating all this solar light and heat on a small area will heat and kill the retinal cells very quickly.

There are much safer ways to view the Sun.

Mount the binoculars on a support (tripod). Cover one of the main objective (sky) lenses and let the light from the open lens fall on a white card. Adjust the size of the Sun's image by moving the card closer to the binoculars. Adjust the focus with the binocular's focus knob. By watching the image on the paper, keep moving the binoculars so that the light passes though the centre of the binocular's eyepiece.

The Project

We do not need fancy instruments to do this project and it can be carried out for either a few months or the entire year.

Find a place well exposed to the morning, noontime and afternoon Sun. Push a stick vertically in the ground. Let it remain at least 0.25 metres (about a foot) out of the ground. Surround the stick with a screen of cardboard or a sheet of material that will not move or be blown away by the wind. A large piece of plywood will work very well.

The Sun will cast the shadow of the stick on the screen. In the morning, the shadow will be long and will extend to the west beyond the stick. A mid day Sun will cast a short shadow towards the north. And in the evening, a long shadow will be cast from the stick to the east. Our project is to record on the screen the position of the shadow at various times of the day and the season.

What Will We Observe?

The shadow of the stick will change quite a bit over the year. In the late spring and early summer, the morning and evening shadows will fall to the SOUTH of the stick. This means the Sun must be north of east at sunrise and north of west at sunset. At noon, a short shadow will extend north of the stick. The Sun is higher in the sky at noon than in the morning and afternoon resulting in the shortest shadow of the day. You may record the direction of the shadow as early and as late in the day as possible. You may also record the direction and length of the shadow at noon. Was the shortest shadow cast at exactly noon, or a few minutes before or after noontime?

On March 22 or September 22 or so, the shadows at sunrise or sunset will point due west and due east, respectively. When did the shortest shadow occur? On March 22, the shadow will be shortest about 10 minutes after 12-noon. And on September 22, this will happen about five minutes BEFORE noon! The actual time will depend on where you are geographically within your time zone.

In March and in September you will notice the shadows are also much longer at noon than in the summer because the Sun is lower in the sky. The extreme case is on the first day of winter on December 22. The explanation of these shadows is shown graphically in the figure below.

But what about the height of the Sun at mid day? The answer to this question rests in the explanation of the "Annalemma", the figure "8" that is usually printed in the middle of the Pacific Ocean on the Earth's globe.

The analemma is a graphical way to depict the time of passage for the Sun across the north-south line in our sky (local meridian). The most obvious source of "error" is the fact that the time zones are course sectors. Someone on the east side of a zone will see the Sun cross the meridian before someone on the west side of the zone.

The second reason is the Earth's elliptical orbit. When closest to the Sun (perihelion) its gravity will pull our planet more quickly around in its orbit. This causes the Sun to appear to move more quickly along the ecliptic around our sky. The opposite is true six months later when we pass through aphelion (farthest point form the Sun).

The third reason is the tilt of the ecliptic to our celestial equator. Where the two lines cross, their angle causes the eastward movement to become foreshortened. The Sun's apparent motion will cause it to cross our meridian before noontime. Similarly, during summer and winter solstice, the apparent movement of the Sun is due east. So at these times the Sun will cross our meridian after noontime.

All this can be better appreciated with this project.