If you caught the eclipse this week, you probably took care to use eclipse glasses or a pinhole camera. But you may have caught a glance at the Sun with the unaided eye. Perhaps while looking at the Sun during totality you saw a bit of the Sun come back before looking away. If you did, you wouldn’t be alone. Lots of people have looked at the Sun before, including Galileo. 

Galileo is most famous for his telescopic observations. His discovery of the the moons of Jupiter and the phases of Venus changed our understanding of the solar system. But Galileo also made naked-eye observations of the heavens. As part of his work he looked directly at the Sun many times. This was not one of Galileo’s best ideas, but he did try to be careful about it. He never looked directly at the Sun when it was high in the sky. He looked when the Sun was low on the horizon at dawn or sunset, often when somewhat obscured by fogs or clouds. When the Sun is low in the sky its light must travel through much more atmosphere to reach us. This not only makes the Sun less intense, it gives the Sun its reddish glow. Galileo found the addition of fog particularly useful, because it allowed Galileo to observe sunspots on the Sun.

Although we don’t know how often Galileo looked directly at the Sun, we know from his letters that he made several observations. He may even have looked at the Sun through a low power telescope. Later, Galileo’s student Benedetto Castelli discovered the projection method for looking at the Sun (using a pinhole camera), which Galileo felt was a far superior method of solar observation. Toward the end of his life, Galileo did go blind, but this was more than two decades after his direct solar observations.

Looking directly at the Sun is always risky. This is particularly true during a total eclipse when the Sun is mostly covered. If you take an extended look at the Sun during a partial eclipse you can damage your eyes permanently. There are several recorded cases of this. But a quick glance isn’t likely to be harmful. If you caught a quick glance during the eclipse, you shouldn’t worry.

And if you still have your solar glasses, you can hang on to them until the next great American eclipse in 2024, or you can donate them to Astronomers Without Borders for others to use.

The post Did You Look At The Sun? So Did Galileo appeared first on One Universe at a Time.

On August 21 of 2017 the shadow of the Moon will trace a path across the United States. It’s a total solar eclipse that many will have the opportunity to observe. But whether you will observe totality or not depends on where you are. Astronomers have been able to predict the path of solar eclipses for millennia, but this new video demonstrates just how precise our predictions have become. 

The video combines lunar terrain data with that of the Earth’s terrain and the predicted positions of the Earth, Moon, and Sun. Data collected from a variety of sources and combined to create an extremely accurate prediction of the 2017 eclipse. Much of this data is publicly available, so you could do your own calculations as well.

If you have a chance to see this eclipse (or any solar eclipse) take it. We already know where you need to be to see it. The only other factor is to ensure a clear sky, but when we get within a few days of the eclipse we’ll have that prediction as well.

The post Wonderful Precision appeared first on One Universe at a Time.

About a year ago Curiosity happened to be in the path of an eclipse. So Curiosity was directed to take a series of photographs of the Sun. You can see the result here.  The shadow moving across the Sun is Phobos, and you can see it pass in real time.  Phobos is must smaller than our Moon, only about 27 km across. It is also much closer to Mars than the Moon is to Earth. Phobos doesn’t completely block out the Sun, but it does block a good chunk of it.

But perhaps most amazing is that we’re now observing solar eclipses on other planets.

The post Mars Transit appeared first on One Universe at a Time.

The diameter of the Sun is about 400 times larger than the diameter of the Moon. The Moon is about 389 times closer to the Earth than the Sun. This means that the size of the Sun and Moon in the sky are about the same, and that happy coincidence is what allows us to have the greatest show on Earth: the solar eclipse.

Having the same apparent size is just the first requirement for a solar eclipse. To see a solar eclipse on Earth, the Moon must pass in front of the Sun from our vantage point. If the orbit of the Moon were exactly in line with the orbit of the Sun, then we might expect a solar eclipse once a month. But the Moon’s orbit is tilted about five degrees relative to the orbit of Earth, and this means that the Moon is often slightly above or below the Sun from our perspective when it passes between the Sun and Earth. So most months there is no solar eclipse.

For the Moon to be lined up in the right way, it has to be located near the plane of the Earth’s orbit when it passes in front of the Sun, and this only happens twice a year, about 6 months apart. This is why there are “eclipse seasons” in Spring and Fall. At least one eclipse occurs during each eclipse season.

Not all solar eclipses are alike. As seen in the image below, since the Sun is larger than the Moon, there are regions where only part of the Sun is blocked by the Moon, and the resulting shadow is called the penumbra. For a much smaller region where the Sun is completely blocked by the Moon, the shadow is called the umbra. The type of eclipse you observe depends in part on whether you are viewing it from the umbra or penumbra.

Credit: Wikipedia

If you are standing in the penumbra, then you will see the Moon block part of the Sun during the eclipse. This is known as a partial eclipse. Since the penumbra is much larger than the umbra, this is the most commonly observed type of eclipse. Sometimes only the penumbra crosses the Earth, and only a partial eclipse is observed, but usually both penumbra and umbra cross the Earth. Most of us only see a partial eclipse if we are lucky, but those standing in the umbra see a very different view.

For those standing in the umbra, the Moon is directly in front of the Sun. One would therefore expect the Moon to completely block the Sun, producing what is known as a total eclipse. Often this happens, but not always. Although the Sun and Moon appear to be about the same size in the sky, their apparent sizes vary slightly. The orbit of the Earth is not perfectly circular, so sometimes it is slightly closer to the Sun, and other times slightly farther away. This means the Sun can appear slightly larger or slightly smaller. Likewise, the orbit of the Moon isn’t perfectly circular, so the Moon can appear slightly larger or smaller as well.

These variations are small, so usually we don’t notice them. But during an eclipse these variations matter significantly. When the Moon appears slightly larger and the Sun slightly smaller, the Moon can completely block the Sun, and viewed from the umbra it appears as a total eclipse. If the Moon is slightly smaller and the Sun is slightly larger, then the Moon can’t completely block the Sun. Instead it mostly blocks the Sun except for a thin outer ring. This produces what is known as an annular (ring) eclipse.

The most rare type of eclipse is known as a hybrid eclipse. In this case the Moon is initially large enough to completely block the Sun, but as the umbra crosses the Earth the relative apparent size of the Sun and Moon shift, so that the Moon is no longer able to completely block the Sun. This means that what begins as a total eclipse changes to an annular one. The reverse is also possible, where an annular eclipse becomes a total eclipse. This means that some people can see a total eclipse, while others can see an annular one.

If you live in the United States and have never seen a total eclipse, make plans for August 21, 2017. On that day a total eclipse will cross the US from Oregon to South Carolina, making it within a day’s drive of most of the country.

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