Not a Sprinkler Arm: Gravity, Orbits and the Turning Galaxy

Watering Through a Dry Summer

1This summer was the driest in years on the plains around Yaretzi Prokopenko's family farm, so the farm's center-pivot sprinkler watered the corn day and night. A center pivot is a long steel pipe held up by towers on wheels. One end is fixed to a well in the middle of a round field, and the whole pipe turns slowly around that point, spraying water as it goes. The farm's pivot is 400 m long and makes one full circle in 24 hours. Yaretzi noticed that the tower near the far end seemed to race along while the tower near the middle barely crept.

2She worked out how fast each tower moves (Table 1). Because the pipe is stiff, every tower goes around once in the same 24 hours, but the outer towers travel around much bigger circles. She checked her numbers by timing each tower with a stopwatch as it passed two fence posts a known distance apart.

3In science class, Ms. Rosamund Iheanacho showed real data for objects that orbit the Sun (Table 2). Nothing solid connects them to the Sun. The Sun holds more than 99% of the mass of the solar system, and its gravity pulls on every planet, dwarf planet and asteroid. The farther an object is from the Sun, the weaker the Sun's pull on it. Ceres is the largest object in the asteroid belt between Mars and Jupiter, and Pluto orbits far beyond Neptune.

4One clear, dry night with no moon, Yaretzi's family saw the Milky Way as a pale band of light stretching across the sky. Ms. Iheanacho explained that the band is our view, from the inside, of the flat disk of our galaxy, which holds hundreds of billions of stars. The Sun is about 26,000 light-years from the galaxy's center. Pulled by the gravity of all the stars, gas and other matter in the galaxy, the Sun travels around the center once in about 230 million years, carrying the whole solar system with it. Stars much closer to the center take less time to make one trip. Even whole galaxies pull on each other: the Andromeda galaxy, about 2.5 million light-years away, is moving toward the Milky Way.