TELESCOPES & SKY

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Telescopes

Apperance & Motion of Sky Questions

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Q1.    Will the full moon be visible shortly after sunset on a clear night? Where will it be? Explain why?  Answer

Q2.    Explain why a first quarter moon sets approximately six hours after sunset.  Answer

Q3.    Describe a simple observation which demonstrates why shadows of Earth cannot cause the phases of the Moon. In general terms, what does cause the phases of the Moon?  Answer

Q4.    Explain why the position of the stars in the early evening sky changes with the seasons.  Answer

Q5.    Describe the apparent motion of the stars in our sky during one night. Why do they appear to move?  Answer

Q6.    Explain why the position of the planets can not be easily predicted for the future. Why are their positions confined to a narrow band of the sky?   Answer

Q7.    What 2 observations show that the seasons can NOT be explained by a changing distance of Earth from the sun?  Answer

Q8.    Why does the Moon change shape in our sky during the course of a month?  Answer

Q9.    A few days after new moon, we see a thin crescent moon. When will it be visible? What will its relation to the sun be in our sky? How do these observations explain its shape?  Answer

Q10.    If a particular star is seen straight overhead in the evening tonight, where will it be in the evening sky in 6 months? In one year? Explain why. Answer

Q11.    Why do the planets never appear in our northern sky?  Answer

Q12.    If Jupiter is visible in the southern sky this evening, where will it be in the evening sky a year from now?  Answer

Telescopes Questions

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Q1.    What is meant by the term "national observatory"? How does one get to use a telescope there?  Answer

Q2.    The human eye, photographic film, and CCD’s are all detectors used in astronomy. What are their relative advantages and disadvantages?  Answer

Q3.    What are the two basic types of telescopes? Why are all large telescopes of just one type?  Answer

Q4.    For astronomical purposes, what are the two most important powers of a telescope? What are their definitions, uses, and limitations?  Answer

Q5.    Why are large refracting telescopes not feasible?  Answer

Q6.    Explain magnifying power and light gathering power for telescopes. Why is light gathering power more important for most astronomical applications?  Answer

Q7.    Describe one of the advances in technology which has revolutionized astronomy in the last decade.  Answer

Q8.    What is a false color image, as used in astronomy?  Answer

Q9.    Describe some (at least two) reasons for placing telescopes in orbit around Earth.  Answer

Q10.    What is meant by the term "light gathering power", as applied to telescopes? What determines how much light   gathering power a telescope has? Why does it matter to astronomers?  Answer

Q11.    What are the two most important reasons for placing telescopes in satellites?  Answer

Apperance & Motion of Sky Answers

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A1.    The full moon will be visible in the eastern sky just after sunset. Full moon occurs when the moon is on the opposite side of Earth from the sun, so that the illuminated side of the Moon is fully visible to us. Because it is the direction 180^o from the sun, it appears on the opposite side of the sky from the sun. Just after the sun has set in the west, the Moon will rise in the east.

A2.    At first quarter the mon has competed one quarter of its orbit around Earth from the time of new moon. This means that the angle between the sun and the Moon in the sky, as seen from Earth, will be 90^o. It will therefore take the Moon an extra quarter of a day, or six hours, to move the quarter of a circle it lags behind the sun in the sky.

A3.    A thin crescent moon is visible in the western sky just after sunset. If that shape were caused by the Moon being mostly in Earth’s shadow, it should be on the opposite side of Earth from the sun, not the same side. The shapes of the Moon are caused by its orbit around Earth. In different positions, we see different portions of its illuminated half.

Many people think seasons are caused by the changing distance of Earth from the sun. Explain why this cannot be the case. Explain what does cause the seasons.

Seasons could not be caused by changes in the distance of Earth from the sun because the seasons in the northern hemisphere are reversed form those in the southern hemisphere (when it is winter here it is summer in the southern hemisphere). Also, Earth is actually closest to the sun in January, when it is coldest in the northern hemisphere.

The seasons are caused by the tilt of Earth’s axis relative to the plane of its orbit around the sun. This tilt causes the sun to be higher in the sky in the summer time and lower in the sky in the winter. When the sun is high in the sky, its light strikes the surface of Earth more directly and is more concentrated. This concentration means that it more effectively heats the surface of Earth. Also, when the sun is high in the sky, its path across the sky is longer, so it stays above the horizon for more hours each day. This gives it more time each day to heat the surface of Earth, producing more warmth on the surface.

A4.    On any given night we see the stars which lie in the general direction away from the sun. As Earth moves in its orbit around the sun, different groups of stars become visible to us as we "point" in different directions.

A5.    During the night, stars rise in the east, move across the sky, and set in the west. This apparent motion is centered around the direction our axis points in space. In the northern hemisphere, that direction is marked by a moderately bright star known as Polaris. Of course, it is really the Earth wich is spinning to cause this motion; the stars themselves are stationary.

A6.    The position of a planet in our sky depends both upon our position in our orbit around the sun and the position of the planet in its orbit around the sun. Because each planet takes a different amount of time to complete its orbit, a given relative position between Earth and the planet repeats on a complicated cycle that is not directly related to our seasonal pattern. However, each of the planetary orbits lies in about the same plane in space. Thus, they are always found in a narrow band of the sky defined by that plane.

A7.    The common notion that the seasons are caused by Earth being closer or further from the sun at different times of the year is not consistent with the observation that the seasons are reversed in the southern hemisphere. When it is winter here, it is summer there. And then there is the fact the the closest approach of Earth to the sun occurs each year in January, in the middle of the northern winter. If seasons were caused by a changing distance to the sun, we would expect to be closest in summertime, not winter.

A8.    The sun illuminates half of the Moon at all times (except during rare lunar eclipses). However, we cannot always see the full illuminated face of the Moon. At any given time, we see only that portion of the illuminated side of the Moon which faces us. For example, when the Moon is between Earth and the sun, the illuminated side is facing away from us and we see no moon (a "new moon"). Two weeks later, after the Moon has moved half way around it orbit, all of the illuminated side faces us and we see a full moon.

A9.    The crescent moon will be visible in the western sky for a few hours after sunset. This means that it is only a few degrees from the sun in the sky, since the sun is just below the western horizon just after sunset. If we consider this situation from "outside" the solar system, the moon will be on the same side of Earth as the sun. While half of it is illuminated by the sun, only a small sliver of that half faces Earth, so we see only a thin crescent.

A10.    A star seen tonight will not be visible in our night sky in 6 months, but will return to the same relative location in one year. These changes occur because the Earth takes exactly one year to complete one orbit around the sun. From each location on our orbit, our night sky consists of those stars that are in the direction away from the sun from that position. In 6 months we will be on the opposite side of our orbit, and will hence see stars in the opposite direction.

A11.    Because the solar system is flat, all the planets follow roughly the same path across the sky. Because Earth's axis is tilted by 23 degrees to this plane, the path of a planet can vary by ± 23 degrees. But even that variation would never place a planet in our northern sky.

A12.    The correct answer is "I have no idea." Because the planets move in their own orbits around the sun, with different periods than Earth's yearly motion around the sun, there is no simple relationship between the current location of a planet and its future position in the sky.

Telescopes Answers

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A1.    A national observatory is one that is available to anyone in the country to use and is supported by the federal government. To obtain time on a telescope at a national observatory, you prepare a proposal that describes what you want to observe and what you expect to learn from the observation. These proposals are evaluated by teams of scientists to determine which are the most scientifically important. The top proposals are awarded free time on the telescopes.

A2.    While the human eye is a wonderfully versatile detector, it is connected to the human brain which often has preconceptions of what will be seen. As a scientific instrument it is unreliable because you cannot show me what you have seen, but can only describe it as seen through the filter of the brain. Images recorded on film can be evaluated by everyone and are recorded without bias, but film is not very sensitive to light. CCD’s are much more sensitive than film, and record their images electronically so they can be processed by computers and shared around the world

A3.    Telescopes can be either reflecting (with a mirror forming the image) or refracting (with a lens forming the image). Large refracting telescopes are impractical because large lenses, which can only be supported around the edge, tend to sag and distort the image. In addition, different colors are bent to a focus at different positions as they pass through thick lenses. Mirrors do not suffer from either of these drawbacks.

A4.    The most important powers of a telescope are light gathering power and magnifying power. Light gathering power is a measure of how bright an image will be in the telescope, and is determined by the diameter of the objective lens or mirror. Large light gathering power is required to make dim objects detectable. The limits are higher cost for larger telescopes and greater engineering difficulties in making precise instruments larger.

Magnifying power determines how large the image will appear in the telescope. It is useful in examining the fine details of resolved objects. Unfortunately, turbulence in our atmosphere sets a limit on useful magnifications for ground-based observations.

A5.    A refracting telescope uses a large lens to gather light and form an image. The weight of a large piece of glass causes it to sag. Because a lens can only be supported around its edge, it is impossible to maintain an accurate surface on a large lens in a refracting telescope. Large pieces of glass also bend different colors by different amounts. This causes color fringes around the images formed by any large lens.

A6.    The magnifying power of a telescope describes how large an image it forms. Light gathering power describes how much light is gathered to form the image. Since stars are too far away for any telescope to magnify them into a discernable image, light gathering power is more important to astronomers because it determines whether a dim object can be seen at all.

A7.    Charge-coupled devices (CCDs) are electronic devices that have replaced film for taking pictures because they are many times more sensitive to light than the best films. Adaptive optics allows astronomers to adjust the image formed by a telescope to compensate for the blurring effect of our atmosphere. Next generation telescopes have very large mirrors that are lighter and more accurate than traditional thick mirrors.

A8.    In a false color image, different colors are used to represent changes in some property of the image. For example, different colors might be used to represent different levels of brightness in a photograph or different colors might be used to represent different elevations in a map showing the surface of another planet.

A9.    Many wavelengths of light do penetrate through the atmosphere at all. The only way to observe them is from above the atmosphere. Even visible light, which does penetrate the atmosphere, is blurred by its passage through the atmosphere. Much sharper images can be obtained from orbit. Some light is absorbed at any wavelength, so images in space are always brighter than those seen on the ground.

A10.    Light gather power defines how much light is gathered by the telescope, and therefore, how bright an object will appear in the telescope. This is important to astronomers because greater light gathering power allows them to observe dimmer objects. The light gathering power is determined by the size (diameter) of the main lens or mirror of the telescope. The greater the area of the main lens or mirror, the greater the light gathering power.

A11.    A telescope in a satellite is above Earth’s atmosphere. This allows it to observe wavelengths, such as the ultraviolet, that are completely absorbed by the atmosphere. Images obtained above the atmosphere are also much sharper than those obtained with ground-based telescopes because they do not have to look through the blurry atmosphere.