Tides
The oceans have tides, but do any other bodies of water have tides as well?
- Only oceans have tides. Even the largest of the world’s lakes is too small to display tidal behavior.
- Larger lakes can have tides as well, although they are measured in inches or fractions of an inch.
- Tides are everywhere. All bodies of water have them in an absolute, theoretical sense (although for a small lake, they would not be measurable). So does the earth’s crust, and so does the atmosphere.
- Tides are everywhere indeed. But even small lakes can have them.
Sunrise, Sunset
Why would any predictions of sunrise and sunset times for your local area, whether obtained from your new GPS display, the Daily Planet, or even the Nautical Almanac, be wrong?
- because they don’t include the effects of the eccentricity of the earth’s orbit
- because they don’t account for orbital eccentricity, and atmospheric refraction
- because they assume a sea level horizon
- They’re not wrong. This is just another blasted trick question!
The Boeing 717
A prototype of the venerable Boeing 707 launched the honored lineage of the Boeing 7-7 series of airliners when it made its maiden flight in July of 1954. The Boeing 727 was introduced into service in February, 1964. Since then, aside from that one gap, the series has continued uninterrupted. The commercial revenue service history of the 737 fleet began in 1968. In late September 1968, the first 747 rolled out of the Boeing factory. Boeing turned its first 757 loose early in 1982. The first 767 emerged from the Everett Washington Boeing plant in August of 1981. In June 1995, the first Boeing 777 entered revenue service. But what the blazes happened to the Boeing 717?
- There actually was a Boeing 717 detail specification, on paper, but its number was altered to patronize the whim of those in power, so a Boeing 717 was never made.
- The numbering sequence was originally intended to increment by 20, and only after the 727 was it changed.
- The number was intentionally skipped to cater to the whim of the then-president of United Airlines, a potential Boeing customer who favored another airplane manufacturer (which is basically choice A), however there actually now is a Boeing 717, although it didn’t see the light of day until after the 777.
- There was a 717, but the prototype was destroyed in an accident during flight testing. There were also no standing orders, so Boeing dropped it, and at that point it doubled its series increment from 10 to 20 (which was, of course, later changed).
The Answers…
Tides
Answer: The sun and moon exert a gravitational pull on the earth (which of course is mutual). However, there is actually a “gradient” of gravity, stronger on the facing side, and which can be thought of as causing a “bulge” of water being pulled away from the earth (and correspondingly on the other side, a second bulge caused by the sun and moon pulling the earth away from the oceans). In between are low tides as water “drains away” toward these high-tide areas. Common sense might convince you that even the largest lake is too small to have tides because the gravitational pull of the sun and moon would act on the entire body of the lake, more or less all at once. There wouldn’t be enough of a source of water (or a space toward which it could move) at points further along the globe to supply tidal flow manifesting this differential. Furthermore, even the land underneath the lake swells under the influence of a high tide, although it is imperceptible because there are no references for the casual observer which aren’t also themselves being pulled by the sun and moon. (Even though it is many orders of magnitude larger, the sun has only about half the moon’s pull because the latter is so much closer.) And yes, the solid earth has tides as well: as much as 18 inches! If you are a pilot, you probably already wondered if the atmosphere is also affected. Guess what? It is. However, because tidal effects are maximized at the equator and decrease rapidly toward the poles, don’t expect that you would be able to discern diurnal barometric changes unless you had a very sensitive instrument, and live near the equator. (Atmospheric tides are about 100 times stronger at the equator.) Atmospheric tides at the mid-latitudes are at the edge of our ability to detect them. At around 10 microbars (0.01 millibars), such small departures are almost nonexistent when compared to weather-related variations in the standard atmospheric pressure of 1013 millibars. Near the equator, an accurate barometer would record daily fluctuations on the order of about 1 millibar. Like oceanic tides, atmospheric tides have cycles, but they come in intervals of 12 solar hours (which is different than the sea’s tides, which are related to the moon’s position). However, all that said, there is a natural phenomenon causing horizontal movement that can be measured for smaller bodies of water. It is called a seiche (pronounced SIGHsh, or also SAYsh). A seiche is the free oscillation of the water in a closed or semi-enclosed basin at a frequency equal to its natural “resonance” period. Seiches are frequently observed in harbors, lakes, and bays, and almost any distinct basin of moderate size. They are usually caused by the passage of a pressure system over the basin, by earthquakes, or by the build-up and subsequent diminution of wind. Following its initiation, the water sloshes back and forth until the oscillation is damped out by friction. For small lakes however, these movements would not be considered “tidal” in nature. The answer is choice C.
Sunrise, Sunset
Answer: Sunrise is when the top of the sun appears on a sea level horizon; sunset is when the top of the sun sinks just beneath it. These times are dependent upon latitude, longitude, and the day of the year. And of course, they also depend on the local terrain. Published sunrise and sunset times are determined without considering the local geography however; they always assume a sea level horizon (yup, choice C). Also, due to atmospheric refraction, whenever you see a sunset, you’re looking three minutes into the past; the sun had already set, three minutes earlier. And when you see a sunrise, you’re also seeing the future (again, about three minutes worth), for the same reason.
The Boeing 717
Answer. If you are familiar with airline history, you probably guessed that the answer is choice C. There was indeed a Boeing 717 specification, and in fact it was used in preliminary negotiations with United Airlines. The president of United at that time was a William Patterson, and he strongly favored Douglas Aircraft. He had even said that he would never buy a Boeing 707 and that the 717 looked too much like a 707. This information reached those at Boeing who were responsible for drawing up the specification documents, and with one quick change to the title page, the 717 disappeared and became a 720 series. Of course, another more modern Boeing 717 appeared, following the merger of McDonnell Douglas and The Boeing Company in 1997. The 717 entered a rigorous flight-test program in September 1998 and received joint certification a year later. The Boeing 717 became the first commercial airplane to receive a “Concurrent and Cooperative Certification” from both the FAA and Europe’s Joint Aviation Authorities. The first 717 customer was AirTran Airways of Orlando, Florida (a.k.a. ValuJet, in a previous incarnation), which took delivery of the first one quite a bit more recently, in September 1999.
