by apodman » Thu Aug 20, 2009 8:51 pm
JohnD wrote:The suns [are reflected], [not] off the rectangular building on the left, but the circular one on the right!
Look at
this link provided in the APOD description. In fact, the multiple sun images are reflected not off the rectangular Pacific Place building on the left, not off the convex curved Conrad International building on the right, but off the convex curved Two Pacific Place building from which the photograph is taken.
JohnD wrote:So [the] panes are not on the same plane, but [form a] convex mirror that shines multiple images on the ground.
Which still doesn't fully explain why the sun reflects onto the scene below from only one spot on each pane.
Redbone wrote:1) Why are there two high tides and two low tides per day?
The tidal force of the moon pulls more strongly on what is closer. The moon pulls up water on the near side of the Earth relative to the solid Earth. The moon pulls up the solid Earth relative to the water on the far side of the Earth. This creates two tidal bulges on opposite sides of the Earth. The solid Earth rotates slightly less than once relative to these two bulges each day.
Redbone wrote:2) Why do we skip one tide every ~28 days?
The moon revolves around the Earth once every ~29 days in the same direction that the Earth rotates, so the moon crosses a given meridian on the Earth only ~28 times in ~29 days. Thus the moon rises and sets nearly an hour later every day, and thus the interval from low-to-high or high-to-low tide is somewhat longer than 6 hours, and the interval from high-to-high or low-to-low tide is somewhat longer than 12 hours. We actually never "skip" a tide; there are just slightly less than 2 (by a factor of ~28/~29) tides per day.
Redbone wrote:3) Why does the high tide occur ~two hours after the moon's rise, well before the moon is directly overhead?
It takes a while for the water (many many gallons) to move in response to the tidal pull. The Earth keeps rotating, and the water never catches up. The continents keep the tidal bulges from traveling continuously around the Earth, so instead the ocean tides slosh back and forth between the eastern and western shores. The time lag varies greatly from one ocean, shore, or inlet to another; that's why we need tide charts. Some locations are completely 90° (6+ hours) out of sync with the position of the moon, some even more. In your example, the tide isn't 4 hours early; it's 8 hours late.
In the case of a river, think of a standing wave in a river rocking from the upstream tidal limit to the ocean end and driven by the ocean water level; picture the tidal waters from the ocean not actually making it way up a river themselves, but the water coming from upstream piling up when the level downstream is high and flowing out when the level downstream is low. I once lived over 100 miles north of NYC on the Hudson River and we had tides right up to the first lock on the Erie Canal. Now I live near Washington, DC which is way up the Chesapeake Bay and then way up the Potomac River from the ocean, and we have tides past both Beltway crossings right up to Great Falls. All of these tides are completely out of sync with the visual position of the moon.
Additionally, when the moon is not new or full, the vector component of the tidal force contributed by the sun is either ahead of or behind the component contributed by the moon, which also changes the lag time forward or backward as we go through a lunar month.
[quote="JohnD"]The suns [are reflected], [not] off the rectangular building on the left, but the circular one on the right![/quote]
Look at [url=http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=two+pacific+place,+hong+kong,+china&sll=22.276776,114.164375&sspn=0.001392,0.001698&gl=us&ie=UTF8&ll=22.277094,114.164434&spn=0.002785,0.003396&z=18]this link[/url] provided in the APOD description. In fact, the multiple sun images are reflected not off the rectangular Pacific Place building on the left, not off the convex curved Conrad International building on the right, but off the convex curved Two Pacific Place building from which the photograph is taken.
[quote="JohnD"]So [the] panes are not on the same plane, but [form a] convex mirror that shines multiple images on the ground.[/quote]
Which still doesn't fully explain why the sun reflects onto the scene below from only one spot on each pane.
[quote="Redbone"]1) Why are there two high tides and two low tides per day?[/quote]
The tidal force of the moon pulls more strongly on what is closer. The moon pulls up water on the near side of the Earth relative to the solid Earth. The moon pulls up the solid Earth relative to the water on the far side of the Earth. This creates two tidal bulges on opposite sides of the Earth. The solid Earth rotates slightly less than once relative to these two bulges each day.
[quote="Redbone"]2) Why do we skip one tide every ~28 days?[/quote]
The moon revolves around the Earth once every ~29 days in the same direction that the Earth rotates, so the moon crosses a given meridian on the Earth only ~28 times in ~29 days. Thus the moon rises and sets nearly an hour later every day, and thus the interval from low-to-high or high-to-low tide is somewhat longer than 6 hours, and the interval from high-to-high or low-to-low tide is somewhat longer than 12 hours. We actually never "skip" a tide; there are just slightly less than 2 (by a factor of ~28/~29) tides per day.
[quote="Redbone"]3) Why does the high tide occur ~two hours after the moon's rise, well before the moon is directly overhead?[/quote]
It takes a while for the water (many many gallons) to move in response to the tidal pull. The Earth keeps rotating, and the water never catches up. The continents keep the tidal bulges from traveling continuously around the Earth, so instead the ocean tides slosh back and forth between the eastern and western shores. The time lag varies greatly from one ocean, shore, or inlet to another; that's why we need tide charts. Some locations are completely 90° (6+ hours) out of sync with the position of the moon, some even more. In your example, the tide isn't 4 hours early; it's 8 hours late.
In the case of a river, think of a standing wave in a river rocking from the upstream tidal limit to the ocean end and driven by the ocean water level; picture the tidal waters from the ocean not actually making it way up a river themselves, but the water coming from upstream piling up when the level downstream is high and flowing out when the level downstream is low. I once lived over 100 miles north of NYC on the Hudson River and we had tides right up to the first lock on the Erie Canal. Now I live near Washington, DC which is way up the Chesapeake Bay and then way up the Potomac River from the ocean, and we have tides past both Beltway crossings right up to Great Falls. All of these tides are completely out of sync with the visual position of the moon.
Additionally, when the moon is not new or full, the vector component of the tidal force contributed by the sun is either ahead of or behind the component contributed by the moon, which also changes the lag time forward or backward as we go through a lunar month.