Category Archives: Astronomy

Recent Asteroid Impacts on Mars and Jupiter

I liked this picture of the recent impact of a small asteroid on Mars. This impact crater is about 100 feet across and was not seen in NASA photographs prior to 2010 and was first seen in a photograph in 2012. The blue color is an artifact of the image enhancement process, which removed the red dust. Continue reading

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Unfortunate Satellite Launch Problem Allows Test of Relativistic Time Dilation

In November 2015, the European Space Agency (ESA) had a launch problem with two of its Galileo navigational satellites that resulted in both satellites being placed into highly elliptical orbits. ESA can burn some of the satellites' station-keeping fuel to bring these orbits back to standard, but this will take some time. While the orbit adjustments are occurring, ESA will use the satellites to provide another test of Einstein's general theory of relativity. Specifically, they will test the prediction that clocks will run slower the closer they approach a massive object. Continue reading

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Percentage of Atmosphere Beneath Observatories

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

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Roche Limit Examples

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

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Challenge of Viewing an Earth-Sized Planet

I listen to audio books during my nightly walks around a local lake. My current selection, Searching for Exoplanets, is one of the best audiobooks I have listened to. The book consists of a series lectures on the state of the search for exoplanets by MIT Professor Joshua Winn. The lectures provide an excellent summary of how astronomers are using remarkably sensitive methods for indirectly detecting the presence of exoplanets circling remote stars. Continue reading

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Floating Habitat on Venus

I have been keeping a close eye on the discussions occurring about sending people to Mars on both one-way and two-way trips. You do not hear similar discussions about Venus because its surface temperature (467 °C) and pressure (93 bar) are too extreme to imagine people surviving there. Continue reading

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Tidal Timing

After a discussion on the annual time shift of the winter solstice, our lunch time topic changed to the topic of tides. During this discussion, I mentioned that tides have a period of about 12 hours and 25 minutes (Figure 1). I will show you how to compute this period in this post. Continue reading

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A Little Solstice Math

We just went through the winter solstice of 2015, which in Minnesota is a subject of celebration. This means that we will now start to see more daylight.

The time of the winter solstice varies each year. It occurs on either 21-Dec or 22-December. One of the engineers in my group asked how the time of the winter solstice moves year-over-year. My response was that it moves forward ~six hours each year until a leap year happens, which resets the cycle. Continue reading

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Venus-Like Exoplanet In the Neighborhood

I have been reading a number of articles that are reporting on a Venus-like planet (GJ 1132b) recently discovered in a nearby star system (Gliese 1132, 12.0 parsecs away). I like to work a bit with the numbers reported in these articles to determine if I actually understand what is being reported. I have to admit that I also like to imagine the day when astronomers are studying Earth-like planets around other stars. I definitely see that day coming. Discoveries like GJ1132b are particularly interesting because astronomers for a long time did not think red dwarf stars were promising for Earth-like planets. Continue reading

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Refraction Error Correction in Sextant Measurements

Refraction is probably the most difficult to understand of all the altitude observation corrections. It is also the most difficult to estimate accurately because it depends so strongly on atmospheric conditions, particularly the rate of temperature variation with altitude (see lapse rate). I will derive in this post a commonly used expression for the refraction correction required for a celestial object with an altitude greater than or equal to 15°. The accuracy of this expression degrades significantly for objects below 15°. Continue reading

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