Chapter 2
What is the angle of Earth's tilt?
23.5 degrees
How long is a lunar cycle?
About 29.5 days, or about a month.
What is the meridian?
An imaginary half circle stretching from the horizon due south, through the zenith, to the horizon due north.
What is the winter solstice?
Around December 21, the moment when the Northern Hemisphere receives the least direct sunlight.
What is the summer solstice?
Around June 21, the moment when the Northern Hemisphere is tipped most directly toward the sun and receives the most direct sunlight
What is the spring equinox?
Around March 21, the moment when the Northern Hemisphere goes from being tipped slightly away from the sun to being tipped slightly toward the sun.
What is the fall equinox?
Around September 22, the moment when the Northern Hemisphere first starts to be tipped away from the sun.
Why do we always see the same face of the moon?
Because of its synchronous rotation; it rotates on its axis in the same amount of time it takes to orbit Earth.
What is angular size?
It is the angle an object appears to span in your field of view.
What is the Arctic Circle?
Latitude 66.5 degrees; the sun remains above the horizon all day on the summer solstice and never rises on the winter solstice.
How do latitude and longitude affect the sky we see?
Longitude does not affect it; latitude affects the locations of the horizon and the zenith relative to the celestial sphere.
Are stars in the same constellation close to each other?
No--they may seem that way on the celestial sphere, but in reality they might be very different distances away from Earth.
What is a sidereal day? Why is it different from a solar day?
One complete rotation of Earth--roughly 23 hr 56 min. Because Earth is orbiting Sun, Sun will not be in exact same place after one rotation--it will be approx. 1 degree off, or about 4 min--which makes a solar day.
How do you know your longitude?
Sun moves at approx. 15 degrees an hour, so you can tell your longitude by how long after the sun passes the prime meridian it passes your meridian.
How can you determine your latitude?
The altitude of the celestial pole in your sky is equal to your latitude.
What is angular distance?
The angle that appears to separate two separate objects in the sky.
What is the reason for seasons?
The combination of Earth's rotation and its orbit, on account of the tilt of Earth's axis; it causes sunlight to fall differently on Earth at different times of the year because its orientation relative to the sun changes over the course of each orbit. The Northern Hemisphere is tipped toward the sun in June and away in December, and vice versa for the Southern Hemisphere.
What direction does the earth spin? What direction does the celestial sphere "spin"?
The earth rotates from west to east, the celestial sphere from east to west.
What path does the sun follow on the celestial sphere?
The ecliptic, which is a circle that crosses the celestial equator at a 23.5 degree angle.
Where on Earth is the latitude 0 degrees?
The equator
How can you mark the dates of the equinoxes?
The equinoxes occur on the only two days of the year on which the Sun rises precisely due east and sets precisely due west.
What is precession?
The gradual wobble that alters the orientation of Earth's axis in space (like a spinning top's axis). It is caused by gravity's effect (from the Sun and Moon) on a tilted, rotating object that is not a perfect sphere.
What is the zenith?
The point in your local sky that is straight overhead, or 90 degrees to the ground.
Where on Earth is the longitude 0 degrees?
The prime meridian (which passes through Greenwich, England).
What is stellar parallax?
The shifting back and forth of stars due to Earth's change of position (undetectable usually because stars are so far away).
What determines angular size?
The size and distance away of the object.
What is a local sky?
The sky as seen from wherever you happen to be standing.
Why does the axis' tilt toward the sun lead to summer, and vice versa for winter?
The steeper angle means 1)more concentrated sunlight 2) the Sun follows a longer and higher path through the sky, which means more hours of daylight
What determines which constellations we see at night?
The sun's apparent location along the ecliptic.
How do people on the equator experience seasons?
They mainly have rainy and dry seasons, with rainy seasons coming when the sun is higher in the sky.
How do circumpolar stars rotate?
They remain perpetually above the horizon, circling around a pole each day.
What two types of regions occur in the shadows of Earth or the Moon?
Umbra, where sunlight is completely blocked, and penumbra, where sunlight is only partially blocked.
What is apparent retrograde motion?
When a planet appears to reverse course and go backward. This occurs when Earth passes a planet in orbit.
What is a lunar eclipse?
When the Earth lies directly between the Sun and Moon.
What is a solar eclipse?
When the moon lies directly between the Sun and Earth (the sun and moon have the same angular sizes).