Monthly Archives: March 2016

I often see popular descriptions of observatories that say things like the observatory "is above 40% of the Earth's atmosphere". I had not thought much about this kind of statement until I saw the Wikipedia's list of the world's highest-altitude observatories, which surprised me as to the height and remoteness of the largest telescopes. I cannot imagine trying to build on these locations (Figure 1 is an extreme example). In some respects, the construction challenges remind me of what builders must have gone through on some lighthouses. Continue reading →

While listening to the audio book The Search for Exoplanets: What Astronomers Know, I heard the lecturer (Professor Joshua Winn) mention the Roche limit and gave a simple approximate formula for evaluating it. The Roche limit provides a lower boundary on how close a satellite may revolve around a planet or star. It is based on the idea that the gravitational and centrifugal forces of the planet work to pull a satellite apart, while the self-gravity of the satellite tends to hold it together. The Roche limit is where these forces are in balance – any closer and the satellite's gravity will be weaker than the centrifugal force plus the planet or star's gravity. Within the Roche limit, the satellite is subject to forces that tend to break it apart. Satellites moving inside the Roche limit are thought to be one way that planetary rings are formed. Continue reading →