Sun Observing Activity

Changing Noontime Altitude | Shadow of the Sun | Altitude of the Sun | Home

Introduction

Astronomy is the study of the heavens and of the grand universe about us. It comes best not from books or people but from our own observations of the sky.

The yearly variation of the amount of daylight and the temperature are some of the most fundamental observed facts of our lives. Everyone is interested in the cycles, yet not everyone understands it.

The seasons are caused by the tilt between the asix of the earth's spin and the pole of its orbit around the sun (ecliptic plane). This has the effect of making the sun reach higher above the horizon and remain visible for longer periods of time near sumer solstice than at winter solstice. Both of the effects mean there is more sunlight striking a given area on the earth's surface during the late spring and early summer. This is why it is hotter than in late autumn and early winter when the opposite is true. Although it would take a full year to record the entire cycle, we can observe part of the phenomenon during the semester.

Changing Noontime Altitude

This activity involves the determination of the sun two-three times a week for two-three months (again weather dependent).

Near noon the sun should be its highest in the southern sky. We need to measure the shadow of the sun.

Each week one member of each team should observe and record the length of the shadow of the Sun.

Measuring the Shadow of the Sun

1. Use a piece of cardboard. From the center of the cardboard draw two lines which are perpendicular to each other and pass through the center of the cardboard. These will indicate the cardinal directions: north, south, east, west. Label the directions. Attach the stick perpendicular to the cardboard where the lines cross.

2. For measuring the altitude of the sun or a bright moon use a meter stick or something else that is straight and of similar length. Set the meter stick flat on the cardboard. Should the cardboard be flat with the horizon? Align the meter stick such that its shadow of the stick falls on one of the lines. Measure the stick (if not a meter stick) and the length of the shadow.

Determine the altitude of the Sun

1. Here the altitude is the angular distance above the southern horizon. To determine the altitude of the sun, take the ratio of the height of the stick (H) and divide it by the length of the shadow (L). This ratio is the tangent of the altitude angle of the object or: H/L = tan(altitude), so that
altitude = arctangen (H/L). Keep a log book of all your data.

2. Once we have the altitude of the sun, we need to determine its declination. The declination is one of the sun's coordinates in the sky, analogous to latitude. The declination of the celestrial equator (by definitions) is 0 degrees. From Bloomington the altitude of the celestial equator is 51 degrees (How do we know this?). To determine the sun's declination, take the sun's altitude and subtract 51 degrees. That is, Sun's declinations = Sun's altitude - 51 degrees.

3. The solstices occur when the sun reaches its maximum or minimum declination. Since the Earth's axis is titled 23.5 degrees, the highest declination the sun will achieve is +23.5 degrees and the lowest will be? These times mark the beginning of summer and winter respectively. Thus, when do spring and fall occur?