Rotation Speed of the Earth

The angular velocity of rotation of the Earth is the same at any latitude from pole to pole. But the linear (tangential) velocity of rotation depends on what latitude you reside, with the maximum tangential velocity being at the Equator. One method of determining how fast the Earth is rotating where you live could come from the difference in Sunrise times between two places with different longitudes. Determining the distance between two locations in the direction of the Earth's rotation, and dividing by the difference in Sunrise-time between those places should enable a good approximation of the local tangential velocity of rotation. However, this is just an approximation.

There are several sources for finding Sunrise, notably your local newspaper. But where do THEY get THEIR information? Researching this information, as well as considering the actual direction of rotation of the Earth as evidenced by Sunrise data from other locations is an excellent collaborative activity! A good source for this kind of information may be found at the US Naval Observatory. Use the Data Services link at this site to calculate sunrise, sunset, moon rise, and moon set for any latitude and longitude you can think of. It also works for most US cities.

### Investigation Questions

We have spent many hours looking at sunrise/sunset data and trying to figure why the times are what they are. After some investigation, we realized that we understand much less about the Earth's orbital dynamics than we originally thought. We have been forced to think about how the tilt of the Earth's axis, the Earth's revolution, and the Earth's orbit all affect sunrise and sunset times. Below are some questions to help you in your research.

#### Questions

• Why are there 6 minutes more daylight than night on the equator during the equinox? Is this true for every day?
• There are two working definitions of noon. One is when the sun is as close to directly overhead as possible. At this point in the day shadows are the shortest. A second working definition of noon is the time halfway between sunrise and sunset.

Use the US Naval Observatory site to check if these two definitions are equivalent. Are they? Would you expect them to be? Does noon occur at mid-day? You can check 'local noon' by marking the time when shadows cast by the gnomon are the shortest. See the circumference page for more detail.
• Why does the United States have less daylight than Canada?
• Here is a question that requires some thought to answer. Look at the sunrise and sunset times for two small cities in Maryland: Salisbury on the Eastern Shore and Accident in the western panhandle near the West Virginia border. The difference in sunrise-sunset for Accident is not the same as for Salisbury. The speed that sunRISE travels from Salisbury to Accident is not the same speed that sunSET travels from Salisbury to Accident. Why is this? You might answer that Accident is north of Salisbury (why?). However, this is not consistent with the observation that sunRISE and sunSET speeds differ on the equinox too.

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