Earth; Moon & Sky Flashcards
1
Q
Explain the Prime Meridian
A
The longitude of the Prime Meridian is defined as 0°.
2
Q
Explain declination and right ascension
A
- Instead of latitude and longitude, however, astronomers use coordinates called declination and right ascension.
- The various markers on the celestial sphere can be used to set up a system of celestial coordinates
3
Q
Explain Declination on the celestial sphere
A
- Measured from the celestial equator toward the north (positive) or south (negative) celestial pole
4
Q
Explain Right ascension (RA)
A
- Is like longitude, except that instead of Greenwich, the arbitrarily chosen point where we start counting is the vernal equinox, a point in the sky where the ecliptic (the Sun’s path) crosses the celestial equator
5
Q
Explain why RA can be expressed either in units of angle (degrees) or in units of time
A
- Celestial sphere appears to turn around Earth once a day as our planet turns on its axis
- Thus the 360° of RA that it takes to go once around the celestial sphere can just as well be set equal to 24 hours. Then
each 15° of arc is equal to 1 hour of time
6
Q
What is another way to locate an object on the celestial sphere
A
To specify its azimuth and altitude
7
Q
Explain Azimuth
A
- Is like a compass direction
- 0° corresponds to the location of true north on the observer’s horizon
- Moving eastward along the horizon, azimuth increases to 90°, 180° and 270° when facing
east, south and west, respectively - Altitude is simply the angle above or below the observer’s horizon a particular celestial object is
at
8
Q
Explain the seasons fully
A
- The difference between seasons gets more pronounced the farther north or south from the equator we travel
- The seasons in the Southern Hemisphere are the opposite of what we find on the northern half of Earth
- Although Earth moves around the Sun along an elliptical orbit, it is not the changing distance to the Sun that causes the seasons
- Distance to the Sun varies by about 3%, yet the seasons (e.g., temperature) vary a lot
- If distance to the Sun were the governing factor, the two hemispheres would have the same seasons
- The seasons are actually caused by the 23.5° tilt of Earth’s axis
9
Q
Explain the equinoxes
A
- Vernal equinox = 21 March
- Summer solstice = 21 June ( leans into the sun at 23.5 degrees )
- first day if summer for northern hemisphere - Autumnal equinox = 21 September
- Winter solstice = 21 December ( leans away from sun at 23.5 degrees )
- first day if summer for southern hemisphere
10
Q
Explain what happens when a hemisphere is pointing toward the Sun
A
- Sunlight strikes it at a more direct angle and is more effective at heating Earth’s surface
11
Q
Explain what happens when a hemisphere is pointing away the Sun
A
- Sunlight strikes it at a more oblique (less direct) angle and is less effective at heating Earth’s surface
12
Q
What is the second thing that determines how effectively the Sun heats the Earth’s surface
A
- The amount of time the Sun spends above the horizon
- Because Earth’s axis is tilted, the ecliptic is tilted by about 23.5° relative to the celestial never thought about astronomy before, we’re sure you have observed that the hours of daylight increase in equator. As a result, where we see the Sun in the sky changes as the year wears on
13
Q
Explain the sun in june
A
- In June, the Sun is north of the celestial equator and spends more time with those who live in
the Northern Hemisphere - It rises high in the sky and is above the horizon in the United States for as long as 15 hours. Thus, the Sun not only heats us with more direct rays, but it also has more time to do it each day
- In the Southern Hemisphere, the June Sun is low in the sky, meaning fewer daylight hours
14
Q
Explain the North Pole and the solstice
A
- Sun’s rays shine down all around the North Pole
- As Earth turns on its axis, the North Pole is continuously illuminated by the Sun; all places within 23° of the pole have sunshine for 24 hours
15
Q
Explain the names of the places that get 24hrs of sunlight at the poles
A
- That circle of latitude is called the Arctic Circle
- In the Southern Hemisphere, we have the Antarctic Circle
16
Q
Explain the Equinoxes fully
A
- Halfway between the solstices
- Sun is on the celestial equator. An observer on the equator will see the Sun at their zenith at midday
- From Earth, it appears above our planet’s equator and favours neither hemisphere
- Every place on Earth then receives roughly 12 hours of sunshine and 12 hours of night.
- The points on the celestial sphere where the Sun crosses the celestial equator are called the vernal (spring) and autumnal (fall) equinoxes.
17
Q
Explain the term solar day
A
The rotation period of Earth with respect to the Sun
- 4 minutes longer than sidereal day
18
Q
Explain the sidereal day
A
- Defined in terms of the rotation period of Earth with respect to the stars
19
Q
Explain sidereal day vs solar day
A
- A solar day is slightly longer than a sidereal day because (as you can see from Figure 4.10) Earth not only turns
but also moves along its path around the Sun in a day - To complete a solar day, Earth must rotate an additional amount, equal to 1/365 of a full turn. The time required for this extra rotation is 1/365 of a day, or about 4 minutes. So the solar day is about 4 minutes longer than the sidereal day
20
Q
Explain apparent solar time
A
- Time reckoned by the actual position of the Sun in the sky
- Indicated by sundials, and it probably represents the earliest measure of time used by ancient civilisation
21
Q
Explain why the exact length of an apparent solar day varies slightly during the year
A
- The eastward progress of the Sun in its annual journey around the sky is not uniform because the speed of Earth varies slightly in its elliptical orbit. Another complication is that Earth’s axis of rotation is not perpendicular to the plane of its revolution
- Therefore, apparent solar time does not advance at a uniform rate
22
Q
Explain lunar phases briefly
A
- Lunar phases follow a 29.5 day cycle.
- The phases of the Moon are not caused by the shadow of the Earth (lunar eclipses are)
- The Moon moves completely around Earth in about 27 days.
- It’s always half of the moon that is lit by the Sun while the other half is in darkness.
- As we watch the Moon from our vantage point on Earth, how much of its face we see illuminated by sunlight depends on the angle the Sun makes with the Moon
23
Q
When is the moon considered new and bright
A
- New = when it is in the same general direction in the sky as the Sun
- Is illuminated (bright) side is turned away from us and its dark side is turned toward its dark, rocky surface does not give off any light of its own. Because the new moon is in the same part of the us.
- Because the new moon is in the same part of the sky as the Sun, it rises at sunrise and sets around us. Since it takes about 30 days to orbit Earth and there are 360° in a circle, the Moon will move about sunset.12° in the sky
24
Q
Explain a solar eclipse
A
The Moon lies between Earth and the Sun, casts its shadow on Earth
***Eclipses can happen only at Full or New Moon
25
Explain a lunar eclipse
- Earth lies between Moon and Sun, casts its shadow on the Moon
- Earth’s shadow falls onto the Moon
26
What are the types of solar eclipses
- Partial
-Annular
-Total
27
Explain each solar eclipse
- If the Moon is slightly smaller than the Sun when the two bodies have their centres aligned an annular eclipse
- If the Moon is slightly closer to Earth than its average distance, and hence appears a bit larger in the sky, it can completely block the Sun - total solar eclipse
- When the Earth, Moon and Sun are slightly mis-aligned, the Earth can lie in the penumbra of the Moon’s shadow - partial solar eclipse
28
Explain ocean tiides
- The tidal bulges on Earth exert a gravitational pull on the Moon.
- Because the Earth rotates faster (once every 24 hours) than the Moon orbit the Earth (27.3 days), the bulge actually speeds up the Moon, pulling it ahead in its orbit.
- This “tidal friction” takes energy out of the Earth and puts it into the Moon’s orbit - making the Moon’s orbit bigger
29
Explain spring and neap tides
- Spring = moon and sun pull in opposite directions ( 180 degrees )
- Neap = moon and sun pull in 90 degrees direction
30
Explain synchronous rotation
- Moon rotates on its axis in exactly the same time that it takes to revolve about Earth
- This is due to tidal locking
- As a consequence, the Moon always keeps the same face turned toward Earth
