Astronomy Chapter S1 Homework

The Sun's declination on June 21st is ______.

+23.5 degrees

Consider the following data for four stars: Star RA Dec. Spica 13hr 24m.1 -11∘20' Deneb 20hr 39m.7 +45∘06' Sirius 6hr 44m.2 -16∘42' Alpha Centauri 14hr 38m.4 -60∘46' Suppose that you are looking at the sky at about midnight on the winter solstice. Which star will be nearest to your meridian?

Sirius (The Sun has a right ascension of 18 hours on the winter solstice, which means that objects with right ascension 6 hours appear on the meridian at midnight. Notice that Sirius is the star with a right ascension closest to 6 hours of the stars on the list.)

A friend says she saw a planet shining on her meridian at midnight. Which planet can you be sure that she did not see?

Venus

Apparent solar time

When the Sun casts the shortest shadows of the day, it is noon according to

If viewing the sky from anywhere on the equator, how much of the Sun's apparent path through the sky would you observe during one 24-hour period?

You would observe exactly half the circular path of the Sun any day of the year.

Suppose you have a time-keeping device that always reads precisely 12:00 when the Sun crosses your meridian. What kind of time is this clock keeping?

apparent solar time

What kind of time can be read directly from a sundial?

apparent solar time

When we say that Jupiter is at opposition we mean that it ______.

appears on the opposite side of the sky from the Sun, so that it is on our meridian at midnight

If you were standing on Earth's equator, where would you look to see the north celestial pole?

on your horizon due north (In other words, the altitude of the north celestial pole would be equal to your latitude on the equator of 0°.)

Due to Earth's rotation, the Sun appears to trace out a complete circle centered on Earth every 24 hours. Imagine viewing the sky from somewhere north of 66.5 degrees north latitude (the Arctic Circle) during the summer solstice (June 21). How much of the Sun's apparent path through the sky would you observe during one 24-hour period?

the whole circular path of the Sun

Standard time

was invented to solve train scheduling problems caused by the fact that apparent (and mean) solar time can differ between locations separated by just a few miles.

Which of the following statements about sidereal and solar days is not true? A sidereal day is the amount of time between successive appearances of any star on our meridian. A solar day is approximately 4 minutes longer than a sidereal day. A solar day is the amount of time between successive appearances of the Sun on our meridian. A sidereal day is the definition of a day on the Earth and a solar day is the definition of a day on the Sun.

A sidereal day is the definition of a day on the Earth and a solar day is the definition of a day on the Sun.

What is the difference between a synodic month and a sidereal month?

A synodic month is the time it takes for a cycle of lunar phases and a sidereal month is the time it takes the Moon to orbit Earth (relative to the stars).

All the following statements are true. Which one explains why our calendar has leap years? Earth's axis precesses with a period of about 26,000 years. Earth's speed varies as it orbits the Sun. A tropical year is slightly more than 365 days. There is a difference between a sidereal year and a tropical year.

A tropical year is slightly more than 365 days.

Consider the following data for four stars: Star RA Dec. Spica 13hr 24m.1 -11∘20' Deneb 20hr 39m.7 +45∘06' Sirius 6hr 44m.2 -16∘42' Alpha Centauri 14hr 38m.4 -60∘46' Suppose you live at latitude 30°S. Which star is circumpolar in your sky?

Alpha Centauri (At latitude 30° S, any star within 30° of the south celestial pole will be circumpolar. Alpha Centauri qualifies (just barely), because its declination of just over -60° means that it is slightly less than 30° from the south celestial pole.)

Which of the following best explains why a solar day is longer than a sidereal day? The fact that Earth's orbit is not a perfect circle causes the length of the solar day to vary during the year, so that it the solar day is sometimes longer and sometimes shorter than a sidereal day. The precession of Earth's axis gradually changes the length of the solar day while leaving the length of the sidereal day alone. Because Earth orbits the Sun at the same time it rotates, Earth must make slightly more than one full rotation between noon one day and noon the next. The Sun rotates much as the Earth does, but more slowly, so that a solar day is longer than a sidereal day.

Because Earth orbits the Sun at the same time it rotates, Earth must make slightly more than one full rotation between noon one day and noon the next.

Consider the following data for four stars: Star RA Dec. Spica 13hr 24m.1 -11∘20' Deneb 20hr 39m.7 +45∘06' Sirius 6hr 44m.2 -16∘42' Alpha Centauri 14hr 38m.4 -60∘46' Which of these stars lies closest to the north celestial pole on the celestial sphere?

Deneb (You can tell that Deneb is closest to the north celestial pole because it has the most positive declination of the stars on the list.)

Which of the following explains why navigators prior to a few hundred years ago found it much more difficult to determine longitude than latitude? Determining longitude requires accurate tables of the celestial coordinates of stars, while determining latitude does not. Determining longitude requires much more precise measurements of angles in the sky than does latitude. Determining longitude requires mathematical techniques that were not known at the time. Determining longitude requires an accurate clock, but latitude does not.

Determining longitude requires an accurate clock, but latitude does not.

As viewed from any place within the continental United States, noon on apparent solar time is defined as the moment when the Sun is ______.

Directly South

Suppose you live at latitude 40°N. Which of the following describes the path of the celestial equator through your sky? It extends from due east on your horizon, to an altitude of 50° in the south, to due west on your horizon. It extends from due east on your horizon, to an altitude of 40° in the south, to due west on your horizon. It extends from due south on your horizon, to your zenith, to due north on your horizon. It extends from due east on your horizon, to an altitude of 40° in the north, to due west on your horizon.

It extends from due east on your horizon, to an altitude of 50° in the south, to due west on your horizon. (No matter where you are located on Earth (except the Poles), the celestial equator extends from due east to due west and passes through your median at an altitude of 90° minus your latitude. It crosses the meridian in the south if you are in the Northern Hemisphere and in the north if you are in the Southern Hemisphere.)

Which of the following best describes the Tropic of Cancer? It is the circle of latitude on Earth for which the Sun appears directly overhead at noon on the spring and fall equinoxes. It is the line of right ascension passing through the constellation of Cancer. It is the circle of latitude on Earth for which the Sun appears directly overhead at noon on the summer solstice. It is the circle of latitude on Earth for which the Sun appears to remain above the horizon for a full 24 hours on the summer solstice.

It is the circle of latitude on Earth for which the Sun appears directly overhead at noon on the summer solstice.

When should we next expect to observe a transit of the Sun by Mars?

Never

It's the summer solstice, the Sun is at your zenith, and you have a UT clock that tells you it is midnight in Greenwich, England. Where are you?

On the Tropic of Cancer near the International Date Line. (The fact that the Sun is on your zenith on the summer solstice means you must be at latitude 23.5°, which is the Tropic of Cancer. The fact that it is midnight in Greenwich means you are on the precise opposite side of Earth (in longitude), which means you are on (or near) the International Date Line.)

Consider the following statements about the Sun's maximum altitude as seen at different locations on different days of the year. 1) If located somewhere in the northern hemisphere, you will see the Sun reach a higher altitude in your sky on June 21 than on December 21. 2) Regardless of your location on Earth, the Sun passes directly overhead every day at local noon. 3) If located anywhere on the equator, the Sun will pass directly over your head every day at local noon. 4) If located anywhere on the equator, you will see the Sun reach a higher point in the sky on June 21 than on March 21. Which are true?

Only statement 1 is true.

daylight saving time

The Sun reaches its highest point closer to 1 p.m. than to noon when our clocks are set to

Which of the following statements is not true about the sky at Earth's equator? The north celestial pole is directly on your horizon, due north (with Polaris quite nearby). Over the course of the year, all 88 constellations will at some point be visible in the evening sky. The Sun will pass directly overhead around noon each day. There are no circumpolar stars in the equatorial sky.

The Sun will pass directly overhead around noon each day.

Leap year

The calendar year has 366 days during a

Suppose Earth's axis had a greater tilt (more than 23.5 degrees). Which of the following statements would not be true? We would have more daylight in summer and less daylight in winter than we do with the current tilt. The celestial sphere would be covered by a different set of constellations than it is with the current axis tilt. The Arctic and Antarctic circles would be located closer to the equator than they are now. Seasonal temperature variations would be more extreme than they are with the current tilt.

The celestial sphere would be covered by a different set of constellations than it is with the current axis tilt.

All the following statements are true. Which one explains why mean solar time differs from apparent solar time? Earth's rotation period is actually about 23 hours and 56 minutes, not 24 hours. The length of a solar day is not always exactly 24 hours. The Sun reaches the meridian at different times at different longitudes within the same time zone. Earth's axis precesses with a period of about 26,000 years. The path of the Sun through the sky depends on both latitude and date.

The length of a solar day is not always exactly 24 hours.

Mean solar time

The length of the solar day is usually at least a few seconds different from 24 hours, so when we use 24-hour clocks we are keeping track of

Universal time

The time in Greenwich, England is also known as

Which of the following best describes the meaning of declination and right ascension? They are terms used by astronomers to describe how the sky changes with the seasons. They are coordinates on the celestial sphere, with declination telling us the angular distance of an object from the celestial equator and right ascension telling us how far around the celestial sphere an object is located from the vernal equinox. They are used to describe the locations and distances of stars that we see in the night sky. They are coordinates on the celestial sphere that are very similar to the coordinates of altitude and direction on Earth.

They are coordinates on the celestial sphere, with declination telling us the angular distance of an object from the celestial equator and right ascension telling us how far around the celestial sphere an object is located from the vernal equinox.

What is the global positioning system (GPS)?

a system for determining positions on Earth using signals from a set of satellites in Earth orbit

The north celestial pole appears 30° above your horizon. The star Vega is on your meridian. By studying your star charts and your clocks, you determine that Vega crossed the meridian in Greenwich (England) 3 hours ago. Where are you?

latitude 30°N, longitude 45° west (The 30° altitude of the north celestial pole tells you that your latitude is 30°N. The information about Vega tells you that your time is 3 hours behind the time in Greenwich, which means longitude 45°W (because each hour of time corresponds to 360°/24 = 15° of longitude).)

Suppose it is January and the Sun remains above your horizon all day long. Where must you be located?

somewhere between the Antarctic Circle and the South Pole (January is summer for the Southern Hemisphere, so for the Sun to remain above your horizon all day long you must be within the Antarctic Circle.)

From a latitude of 60 degrees north during the winter, the Sun will ______.

spend most of each day below the horizon (During winter at high latitude, you will have short daylight and long night.)

Our modern calendar is also known as ______.

the Gregorian calendar

Universal time, or UT, is ______.

the mean solar time in Greenwich, England

Suppose you know the declination of a bright star that is now crossing your meridian. What else must you measure in order to determine your latitude?

the star's altitude