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River Science: Celestial Tracking

In truth, the tracking of celestial objects is as much shipcraft as river science. In 1609 (and indeed, for thousands of years before that and to this very day), celestial navigation was a crucial aspect of nautical life.

We use a traditional instrument called a quadrant to measure the angle of celestial objects above the horizon; these include the sun and various stars.

Captain Hudson himself would have used this instrument to determine his latitude (his position on the north-south axis of the globe). The quadrant is so named because it forms one quarter of a circle (a 90-degree angle). 0 degrees represents the equator, while 90 degrees represents the north pole. Gravity pulls the dangling plumb bob straight down toward the center of the Earth. A navigator in the northern hemisphere can look up along the quadrant's flat edge to observe Polaris, the north star; the resulting angle indicated by the plumb bob would then indicate the user's northern latitude (see the Daily Logs for our latitudes at each anchorage throughout the voyage).

By tracking the sun, we can determine local true noon (the point at which the sun is highest in the sky), which can differentiate considerably from the local standardized time, depending on Daylight Savings Time and one's location in a time zone. In the northern hemisphere, the direction of the sun at local true noon in turn determines due south. Thus, a celestial navigator could use a quadrant and the sun to calibrate a compass.

On this leg of the Voyage of Discovery, the research team of Cassie, Kevin, Ntando, and Peter studied celestial tracking for their presentation project.

Of course, overcast skies can prove a serious annoyance, since they hide a navigator's celestial reference points. This holds true whether you're just trying to observe the sun for a project or a sailor lost at sea in the Northern Atlantic. Fortunately, we enjoyed clear skies during our data collection on Day Three.

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