A Solar Buoy on the River: Winter Light and a Moon That Keeps One Face

The Buoy at Clearwater Bend

1The Clearwater River Alliance, a group of volunteers and students from Fallbrook Middle School, spends many Saturday mornings pulling trash out of the river. This spring the group anchored a small floating buoy in a quiet bend to track how warm and how cloudy the water is. The buoy sits at 45° north latitude. Its sensors and radio run on a battery that is charged by a flat solar panel on top of the buoy, and the system needs about 35 Wh (watt-hours) of energy each day. Eighth grader Oriana Petrakis checks the buoy's data for the group.

2Oriana listed the panel's energy on clear days through the year, along with sky-app data for the buoy's location (Table 1). She also found that Earth is about 147 million km from the Sun in early January and about 152 million km in early July. Earth's axis is tilted about 23.5°, and it keeps pointing toward the same place in space, near the North Star, all year.

3To test how the angle of light matters, crew leader Mr. Desmond Achterberg helped Oriana set up a lamp test at school. A small solar cell lay flat on a table in a dark room. The lamp moved along a curved track, so it always stayed 50 cm from the cell but shone from different heights above the table, from straight overhead (90°) down to low at the side (20°). The lamp, the bulb and the cell stayed the same in every trial. A meter measured the electric current from the cell (Table 2).

4The buoy also carries a small camera that takes a picture of the night sky every hour. Over five months, every photo of the Moon showed the same pattern of dark patches. One clear Saturday, the crew arrived at dawn and saw a half-lit Moon high in the southern sky, lit on its left side. Months later, the camera caught a total lunar eclipse: around midnight, the full moon slowly turned a dark coppery red.

5To explain the photos, Mr. Achterberg used a model in a dark room. A lamp stood for the Sun, Oriana's head for Earth and a foam ball on a stick for the Moon, with a sticker on one side of the ball. Oriana moved the ball counterclockwise around her head and kept the sticker facing her the whole time. To do that, she had to twist the stick slowly in her fingers, and by the time the ball was back where it started, it had turned around once. In the real sky, the phases repeat about every 29.5 days. Seen from the Northern Hemisphere, the first quarter moon (right half lit) comes about a week after new moon, the full moon about two weeks after, and the third quarter moon (left half lit) about three weeks after.