1. Its appearance in 1066 sent religious and non-religious people running into churches. Halley's Comet was considered a .......... of doom.

2. The comet was blamed for earthquakes, plagues, and the births of two headed farm animals in Switzerland. A mysterious red rain was interpreted to be an .......... warning of the end of the world.

3. English astronomer, Edmond Halley for whom the comet was named, proved that the comet would be a .......... event approximately every 75 years.

4. Halley worked backwards in determining .......... historical dates. A passage in the Book of Joshua describing the "sun standing still in the sky" in reference to sighting a comet, meant Israelites were living in the land of Canaan in 1404 BC and had observed the event.

5. Since many events in history are not well documented, it is next to impossible to determine when they actually occurred. However, astronomy is more exact: solar eclipses can now be backdated using .......... accounts.

6. For example, historians can put dates to ancient events simply by using the .......... of the earth and moon orbits. A five-year war between the Lydians and the Medes was known to have stopped when a total eclipse occurred on May 28, 585 BC.

7. Stardating was another process used to determine construction dates of the Great Pyramids. The Great Pyramid of Cheops is .......... to within a tenth of a degree, built to face true north.

8. True north was determined by observing the stars, Mizar and Kochab, follow the same axis of rotation as the earth. A plumb line cut between them would continue the .......... to determine true north.

9. One .......... in this calculation is that stars change position relative to the earth. The true north alignment in 2467 BC changed. Polaris is the star now aligned with true north.

10. Nevertheless, astronomers concur that pyramids built after Cheops are all slightly misaligned by a small degree toward the west. When this .......... was measured, the construction date of the Great Pyramid was determined to be 2478 BC.