Astronomy: Selected Topics

Chapter 1

Earth

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The Amazing Global Positioning System (GPS)

Many people in many fields use the Global Positioning System

1.     Military officers. The U.S. Dept of Defense, which developed the GPS, can keep track of troops, target cruise missiles and precision-guided bombs, and detect nuclear explosions anywhere in the world.

2.     Navigators of cars, planes and ships rely on GPS. Glider pilots and mountain climbers also use it.

3.     Surveyors use GPS to locate boundaries and surveying markers. Specialized road construction devices are even capable of manipulating blades and buckets of equipment for grade control by GPS.

4.     Some scientists and engineers need precise time references. The GPS time signals are used to set time-code generators and NTP (Network Time Protocol) clocks for synchronizing widespread computer networks.

5.     Geophysicists and geologists. Example: GPS reference time signals can record the arrival time of earthquakes at various recorder locations very accurately.

6.     Mobile phone engineers. Mobile phone locator signals that use conventional radio are not nearly as accurate as GPS locators are, so mobile phone designers can get by with less powerful transmitters by using ancillary GPS.

7.     Employers of traveling employees can keep abreast and help them. For example, a drug company sales manager can see that one of his salesmen is presently calling on a CVS drugstore in Lakewood, Ohio.

8.     Husbands and wives can track spouses and others.

9.     Care-givers can monitor elderly people and know where to look for them by contacting a GPS device carried in their clothing. Pets that are equipped can also be located.

10.     Parents keep track of children. It works so long as the kids don’t leave their GPS device somewhere else.

11.     Car and truck fleet owners can track their vehicles. GPS can also control harvester vehicles as they work in fields.

12.     Police can find stolen autos and fugitives. Police are now equipping decoy cars with GPS devices to track and capture car thieves. Detectives installed a hidden GPS device in murder suspect Scott Peterson’s car, and they knew whenever he drove past Amber Frey’s house, knew he was at the bay to observe the body searches, and knew that he drove south toward Mexico, so they arrested him in San Diego.

13.     Construction executives track stolen equipment. Between $300 million and $1 billion dollars worth of heavy equipment is stolen every year. Only 10 % is ever recovered. With a hidden GPS, the police can retrieve it, if loss of the equipment is reported before it reaches a border or a port.

14.     Aircraft passengers can follow their locations. Systems are now being installed to permit passenger GPS to be used even when landing and taking off.

15.     Geocoaching and orienteering hobbyists. Objects are first hidden by someone; other geosearchers then try to find them using hand-held GPS receivers.

16.     Hikers and nature walkers use GPS to avoid getting lost or to help rescuers find them.

The GPS has 24 satellites. There are six orbits with four satellites in each orbit. They circle the Earth twice every day at 12,600 miles altitude. There are also three spare satellites up there, for use in case of failures.

Data received from the satellites can be merely stored at the tracking receiver or be retransmitted at regular intervals to a central location or to a computer by two-way radio or communications satellite.

The accuracy depends on several factors, including the receiver quality, but it is usually about 50 feet. The best achievable accuracy of a standard GPS receiver is about ten feet, and special systems can be accurate to one foot! Clocks on the satellites are compensated before they are launched for the difference in how fast the clocks will run due to the theory of relativity at the satellites’ high speeds.

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Another Angle on Sunsets

At sunset, when the bottom edge of the Sun appears to just touch the horizon, the Sun is actually entirely below the horizon, with its top edge geometrically at the horizon! Refraction in the atmosphere causes the rays from the Sun to bend downward by about 1/2 degree, which happens to be the size of the Sun as seen from Earth. As a result, the entire Sun is still in sight even after it is entirely below the horizon.

Refraction also makes the bottom of the Sun look flattened at sunset. That’s because the atmosphere is denser at lower altitudes. Light from the top edge of the Sun is refracted only slightly at sunset, and it curves downward only a little, but light from the bottom edge is refracted a lot more, so it has great downward curvature. Sighting along that downward curvature makes the bottom of the Sun look higher than it is, relative to the top of the Sun.

Then why doesn’t the horizon itself have the same refraction as the Sun, so that the Sun would appear to be in the right place? The optical path of the horizon to the eye doesn’t experience any change of refractive index of the media it traverses. Its entire path is along the surface of the Earth, where the density of the air is uniform. Sunlight, on the other hand, comes from outer space into the Earth’s atmosphere. Like a beam of light going from air into a glass of water, the Sun’s rays change direction when they enter the atmosphere.

Incidentally a sunset looks exactly like a sunrise, except that in the morning the atmospheric conditions are often different than at night. The Moon is refracted just like the Sun.

The Sun’s image appears to be about the same size every day, because the Earth’s orbit is not very eccentric (0.0017). The Moon is a little different. Someone might say The Moon is big tonight. Actually, although the Moon’s apparent diameter fluctuates more than the Sun’s, it is not much more. The Moon’s distance from the Earth varies from 221,000 miles to 253,000 miles, which is 13.5%.

But the Moon also has phases, and a full Moon probably looks larger because it is so bright. Like the Sun, the Moon subtends about 1/2 degree at the eye on average, as is demonstrated during a total lunar eclipse of the Sun.

When there is a crescent Moon we can still see the dark part dimly. That’s because sunlight reflected from the Earth to the Moon illuminates the dark part of the Moon, and some of that Earthlight is then reflected back to the Earth. The Moon reflects only 7% of the light that strikes it, while the Earth reflects five times that percentage. That’s because the Earth has white clouds and the Moon doesn’t.

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The International Space Station (ISS)

It’s a long time to stay away from wife and children. Six months is the average time for an astronaut to spend in the International Space Station. There were three crew members at first, but now changed to six, and the tours of duty of the six crew members will overlap. The windows are covered during their sleep hours to simulate nighttime. Otherwise they would see sixteen sunrises every twenty-four hours. The crew wakes up at 6 AM and inspects the craft, after which they have breakfast, review the day’s plans by radio with Mission Control, and start work about 8 AM.

They work six days a week and exercise several times a day. They do scientific experiments such as studying the effects of space environment on humans. The results are expected to make lengthy space flights and space colonization feasible some day.

The International Space Station is the only space station presently aloft. Unlike most satellites, a space station doesn’t have major propulsion or landing facilities, so other vehicles have to take things up and build it in orbit.

ISS was launched on Nov. 20, 1998, and after ten years portions were still being assembled by supply missions. With 18 missions as of March 2009, it was about 81% complete. Completion is expected in 2011. It has had people on board since November 2, 2000. It uses solar panels for energy.

ISS is in a low orbit of 190 nautical miles above the surface of the Earth, where the crew is protected from solar flares by the Earth’s magnetic field. Traveling 17,000 miles per hour, the ISS orbits the Earth every hour and half. An observer on Earth can see it at night with the naked eye if the station itself happens to be in Sunlight.

The ISS is the largest space station so far. It is 240 feet long and weighs 611,000 lbs. Large-scale space cities are being talked about but cannot be built in today’s political climate because the launches are too expensive.

The cost of the ISS is expected to be $35 billion to $100 billion, spread over a period of 30 years. ISS is a joint venture of fourteen nations, including the USA. George H. W. Bush and George W. Bush supported it, and President Obama is expected to support it.

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Meteors, Meteorites, Meteoroids, Asteroids and Comets

Meteors When a meteoroid enters the Earth’s atmosphere and starts to burn and glow from friction it becomes visible as a meteor. A meteor is a streak of light seen in the night sky when a meteoroid burns itself out. The best observations are at four AM local time. Under clear conditions the eye can see about ten per hour. Most visible ones have magnitudes in the range + 3.75 to + 0.75. A meteor shower is a number of meteors with approximately parallel trajectories.

Meteorites Meteor fragments that survive the atmospheric fall are called meteorites. They are interplanetary objects that fall all the way to the Earth’s surface. The first documented one was a stone that fell in Europe in 1492, but it wasn’t until 1803 that the scientific community accepted meteorites as being extraterrestrial. Roughly six observed falls and ten much later finds are added to the list annually. About 3300 hit the Earth each year, but most land in oceans, deserts and other uninhabited regions.

Meteoroids Meteoroids are chunks of rock in space, which are smaller than a planet or asteroid but larger than a molecule. Most of the meteoroid cloud around the Sun has particles smaller than 0.001 gram. Meteoroids are usually produced by the decay of short-period comets and the collisions of asteroids.

Asteroids Very large chunks of rock in space, some as large as mountains, are called asteroids. Thousands of asteroids orbit between Mars and Jupiter, and a few have orbits that bring them near the Earth.

Comets Comets are different from asteroids in that they contain significant amounts of ice that vaporizes and glows when exposed to the light and to charged particles that stream out from the Sun. Comets are much larger than meteors and don’t have to enter the Earth’s atmosphere to be visible. A meteor streaks across the sky in a few seconds or less, but comets are visible for weeks as they slowly move against the background stars.

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Asteroid Risks To Human Life

There is a possibility that an asteroid will wipe out humankind, as happened to the dinosaurs. But we can still sleep tonight. The dinosaurs didn’t have computer-assisted tracking or guided missiles.

Definitions

Tunguska A remote area in Siberia where in 1908 an asteroid (or comet) exploded 3 miles above the Earth. It caused physical damage comparable to the bomb at Hiroshima although the object was only about 130 feet wide. Reindeer herders 50 miles away were knocked down.

Asteroid A rocky body without atmosphere that orbits the Sun but is too small to be a planet.

Asteroid belt A region between Mars and Jupiter where about 95% of known asteroids circulate. Some maverick asteroids come inward and cross the orbit of the Earth.

Large asteroids Among those asteroids that cross Earth’s orbit, there seem to be only 1000 or 2000 that are a mile wide or more. The devastating Yucatan asteroid of 65 million years ago was 5 miles wide. A large one could hit the Earth tomorrow, but maybe not for a thousand years. We’ll probably be able to defend ourselves better 20 or 30 years from now.

Small asteroids Asteroids that are less than 150 feet across are much more numerous. The orbits of a hundred thousand of them cross Earth’s path, and although small they can be deadly.

Impact Damage depends on whether the object strikes Earth on land or ocean. An ocean impact is worse and 70 % of Earth’s surface is ocean. A tsunami wave caused by an asteroid could be a hundred times taller than the recent Earthquake-caused tsunami that killed thousands of people. If it hits on land, an asteroid only 500 feet wide could destroy a large urban area. A mile wide-asteroid could immediately wipe out all life in a large part of the Earth near where it strikes, and severely disrupt the rest of the Earth. The dust it raises could block the Sun long enough to kill food crops and create worldwide famine.

Detection methods Telescopes on Earth can detect large asteroids but can’t detect objects as small as the 130-foot Tunguska one until they are dangerously close.

Protection Earth can be protected from an approaching asteroid by either destroying or deflecting it.

Destruction This method requires breaking the asteroid into small enough pieces that it will burn up in our atmosphere. An intruding asteroid could be blown apart by a nuclear bomb.

Deflection This method requires that an approaching asteroid be deflected perpendicularly to its trajectory. The Earth’s radius is 4000 miles, so the new path preferably misses the center of the Earth by 5000 miles or more. An asteroid could be deflected by exploding a nuclear bomb near it, by striking it with a missile of great momentum or by landing a thrusting device on it and pushing it away.

Deflection by thrusting The earlier a pulse of deflecting thrust is applied, the less thrust is required, because there is more time for the altered trajectory to diverge before it gets near the Earth. If the asteroid were pushed slowly and steadily for several months until it passes us, a greater thrusting acceleration would be required to ensure a safe clearance.

Forecasts Cosmologists think that a billion years from now the Earth’s environment probably won’t be habitable for humans because of solar system aging. Nostradamus, a sixteenth-century French physician and astrologer, prophesied that the world would end in the year 3797. The Bible also predicted an end, but didn’t say when: Heaven and Earth will pass away...But of that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father alone. Mathew 24:35-36. Some astronomers think it is quite possible that an asteroid will destroy civilization within the next million years.

Risk The chances of our dying by asteroid are estimated to be the same as those of dying in a plane crash, i.e., about one in twenty thousand. There are other threats to worry about that are more likely.

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Will We Become Extinct Like Dinosaurs?

Scientists are pretty sure that a meteorite landed about 65 million