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1
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- Continue Ch. 3: Cycles in the Sky
- Seasons
- Angular Size formula
- Eclipses
- Phases of the moon (Friday)
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2
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- Plot position of Sun relative to stars, over one full year.
- Complicated by fact you can’t see Sun and stars at same time.
- Once you have full map of sky, you can work this out by seeing what
stars are opposite sun 12 hours later.
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3
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4
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5
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6
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7
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- Seasons almost entirely due to TILT of Earth
- Seasons opposite (not the same) in N & S Hemispheres
- Earth’s orbit slightly elliptical
- Slightly closer to the sun in N. Hemisphere Winter
- But this changes as tilt precesses in 26,000 yr cycle
- Expect N. Hemisphere winter to be slightly milder
- Positions of continents and oceans actually more important
- Effect is important for Mars -- more elliptical orbit
- Cyclic variations in climate as tilt precesses (and tilt and ellipticity
also gets slightly larger and smaller
- VERY IMPORTANT TOPIC (Re: Global Warming)
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8
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9
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10
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11
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12
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13
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14
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- How big is the moon? Have you
ever seen the moon near the horizon?
Has it looked huge, much larger than when it is high in the sky?
- Last full moon was Jan. 7. Next
one is Feb. 6. Have a look. After we do phases (next time) will
know where and when to look!
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15
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16
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17
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18
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- Longest possible total eclipse is only 7.5 minutes. Average is only 2-3 minutes.
- Shadow sweeps across Earth @ 1000 mph!
- (Compare with scene in The Mummy Returns!)
- Birds will go to roost in a total eclipse. The temperature noticeably drops.
- Totally predictable (even in ancient times, e.g., the Saros Cycle,
eclipse pattern repeats every 6585.3 days or 18 years, 11 1/3 days).
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19
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20
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21
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22
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Notes
