The Earth rotates
We see the Sun rise and set, and it seems as though it's going around the Earth. But how would you show that the apparent movement of the Sun is created by the Earth's rotation?
Nineteenth-century French scientist Léon Foucault devised a pendulum to demonstrate that it's the Earth rotating on its axis that accounts for the Sun's apparent movement. Google honored Foucault with a doodle on September 18, 2013, his 194th birthday. The video shows how the interactive doodle let people experiment with a virtual model of a Foucault pendulum. You may have seen a real one in a science center.
A freely swinging pendulum can't spontaneously change the direction of its swing. This can only happen if you apply a force to it. Yet the pendulum seems to be moving in a circle. If it can't do that, what could give you the same effect? Easy. It's also what you see when Earth is rotating under the pendulum.
In the video you can see two sliders being used, showing their effects. The one on the left changes the speed of the swing. The other one changes the latitude, because the behavior of a Foucault pendulum varies with latitude.
A lunar eclipse as it happened
On June 15, 2011 a total lunar eclipse with 100 minutes of totality took place. It wasn't visible from the USA, so with some assistance from the SLOOH Space Camera, Google showed the eclipse on its homepage in nearly real time. A slider allowed viewers to view the changes in the Moon's appearance.
The speed of light
Light travels so fast that it seems to get from place to place instantaneously. Yet even centuries ago some people wondered if it had a measurable speed. Galileo and others had some ideas about testing it, but it was Danish astronomer Ole Rømer who followed up an idea and put in the observing time.
An animated doodle appeared on December 7, 2016, the 340th anniversary of Rømer's determination of the speed of light. It shows Rømer pacing the observatory floor as his pendulum clock kept time. The two "O"s also link the Earth with Jupiter and Io.
What Rømer was doing was timing the eclipses of Jupiter's moon Io. The timings weren't always the same, and he thought that was because the light from Jupiter took different amounts of time to get to Earth from Jupiter, depending on the distance between them. His observations supported the idea that light had a definite speed. He followed it up by trying to work out what the speed of light was. Rømer's rough estimate was about 25% slower than the real value, but this was amazingly close considering the instruments available in the 17th century.
Ultracool dwarf and the seven planets
On February 23, 2017 NASA announced the discovery of a star with seven terrestrial planets, three of them in the star's habitable zone. A doodle appeared on the same day showing the TRAPPIST-1 system.
NASA's Spitzer Space Telescope had studied the system in the infrared, establishing the planets' sizes, distance from their star, and where possible, an approximate mass and density. However the telescope in the animation isn't a space telescope. It represents ground-based telescopes. The TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) project, located at the University of Liège in Belgium, searches for terrestrial planets. They discovered the cool red dwarf star with the TRAPPIST telescope in Chile and named it TRAPPIST-1. They also found evidence of at least three planets, and several telescopes in addition to Spitzer were used to find and study the star's enitre planetary family.
Asteroid 2012 DA14 – a miss
Late on February 14, 2013, Google's home page showed the doodle that would be going out the next day. Astronomers were excited about asteroid 2012 DA14 making a very close visit to us. It would be a good opportunity to see an asteroid at fairly close range. The 30-meter asteroid, at its closest, would be at an altitude of 27,600 km (17,150 mi), which is closer than some communications satellites.
In this video you can see the asteroid 2012 DA14 doodle. It's cute the way the Google "g" scooted out of the way to leave a gap for the asteroid.
But something completely unexpected happened. It was the morning of February 15th in Chelyabinsk, Russia when a 20-meter asteroid exploded in the atmosphere. It created shock waves that did considerable damage to buildings, which in turn caused hundreds of injuries, mainly from flying glass and other debris.
It was still February 14th in the USA when the reports started coming in from Russia. The effects of the uncatalogued asteroid made an asteroid celebration seem inappropriate, and Google removed the doodle. Later in the day, 2012 DA14 would miss Earth, as predicted. The doodle wasn't much missed, as most people didn't know it had ever been.
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