Lab5F11Tides - GE 70A: Evolution of the Cosmos and Life...

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GE 70A: Evolution of the Cosmos and Life Name:_______________________ LABORATORY/DISCUSSION WORKSHEET Earth's Motion and the Tides Part 1: The Celestial Sphere OBJECTIVES: Familiarize yourself with the equatorial coordinate system that astronomers use to describe the position of objects on the sky Introduction The Greeks believed that the Earth stood still and that an imaginary sphere which contained all the heavens rotated around the earth with a 24-hour period. Of course we now understand that the Earth does not stand still with the sun and stars circling around it; it is the Earth which rotates so that the sun and stars only appear to move around the earth. Yet even though the Greeks were wrong, the visualization of the night sky as a rotating celestial sphere is still used to help astronomers understand the motion of the Sun and stars as viewed from Earth. Figure 1: The Celestial Spher Just as longitude and latitude are used to navigate on the spherical Earth, lines of Right ascension (RA) and declination ( DEC) are used to navigate on the celestial sphere. The lines of RA, like longitude lines, run from pole to pole, and the lines of DEC, like latitude lines, run parallel to the equator. In other words, RA and DEC lines are just the Earth's longitude and latitude lines projected onto the celestial sphere. Like the Earth, the celestial sphere has a celestial equator and north and south celestial Poles , or NCP and SCP, respectively, which are just the projections of these points on Earth onto the sky. Therefore, the North Celestial Pole is that point in the sky directly above the North Pole on Earth. A declination of 40° N passes directly overhead at latitude 40° N. RA and DEC coordinates can be used to plot the positions of stars and heavenly objects on the celestial sphere. The Ecliptic The path that the sun traces out on the celestial sphere in the course of a year is called the ecliptic . Because the Earth's axis is tilted 23.5°, the ecliptic is tilted with respect to the celestial equator, which is just the projection of Earth’s equator (Figure 2). In the second section of this lab, we will see how this tilt causes the seasonal changes we experience on Earth. Figure 2 shows the path the sun appears to travel around the Earth from the Earth's perspective. Examine Figure 2 closely. Twice during the year, in the spring and the fall, the sun is directly over the equator, and sunlight strikes the whole surface of the earth from pole to pole. On these 2 days, the day/night length is 12 hours everywhere.
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1. What do we call these 2 days when the sun is directly overhead at the equator ? [Hint: the name is Latin for "equal night"] Figure 2: Visualizing the Ecliptic Some Conventions Just as the convention for measuring latitude on the Earth is to start from the Earth's equator, the convention for measuring declination of a celestial body on the celestial sphere is to start from the celestial
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Lab5F11Tides - GE 70A: Evolution of the Cosmos and Life...

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