Techniques from radio astronomy are being used to measure the motion of the earth's continents. Very long baseline interferometry (VLBI), normally used to measure the positions of celestial radio sources, also measures the position of radio telescopes to a few centimeters. VLBI has determined that the rate of slippage along the San Andreas fault, and the rate at which the Atlantic Ocean is widening, is from 1 to 3 cm per year.
Astronomers also study the geology of other planets, thereby giving geologists a better perspective on terrestrial problems. Examples include insights into vulcanism and tectonic mechanisms obtained by comparing Earth to Mars, Venus, Io, and Triton.
The energy from the sun is a critical parameter determining the habitability of the earth. There are tantalizing hints of terrestrial climatic changes on long time scales driven by changes in the solar luminosity and in the earth's orbit around the sun. We need to understand solar effects on our environment in order to isolate and better understand human perturbations on the environment. Measurements from space during the most recent solar cycle show that the total brightness of the sun changes about 0.1 percent between maximum and minimum activity (Figure 8.2). Most models suggest that such changes should cause a minor global temperature change of less than 0.1°C. However, the integrated solar activity, smoothed over several cycles, has increased roughly in phase with the apparent 0.5°C warming trend of the past century; the Little Ice Age in the 1600s coincided with an extended period of exceptionally low solar sunspot activity. We need to understand more about climatic cycles on both the sun and the earth to determine how significant these effects are. A promising way of understanding solar cycles is to compare the sun with the other stars known to have 5- to 20-year cycles.
There are numerous short-term effects of sudden increases in the ultraviolet, x-ray, and particle radiation coming from the sun. The high-energy radiation from such flares hits the earth's upper atmosphere, causing heating and ionization, and also modifies terrestrial electric and magnetic field structures on the ground. The 11-year cycle of solar activity reached near-record levels in 1989. Documented effects of large solar flares include major power outages, disruption of radio communication, and added drag on satellites. Continuous monitoring of the sun can give a few hours warning of the arrival of particles from solar flares. These observations will be important for the safety of astronauts inhabiting, or traveling to, the moon or Mars.