Lesson 2 Flashcards
Define what a constellation is?
A region of the sky with well-defined borders (the patterns of the stars help us locate the constellations)
How many official constellations are there?
88
What is a galactic plane?
Our galaxy’s disk of stars
Why do stars and constellations appear to surround Earth in a sphere?
Due to our lack of depth perception in space
What is the celestial ecliptic?
It is the sun’s apparent annual path around the celestial sphere
In which Hemisphere are the widest and brightest parts of the milky way visible?
The Southern Hemisphere
What is the local sky?
The sky as seen from where you happen to be standing
From which position do we view the universe in relation to the Milky Way?
We view the universe from our location a little more than halfway out from the centre of the Milky Way
How can we pinpoint any object in the local sky?
By stating its direction along the horizon (azimuth) and it’s altitude above the horizon
What is the azimuth?
Degrees clockwise from due North
What are the approximate angular sizes of the sun and moon?
About 0.5 degrees
How can we further be precise when stating an object’s angular size?
By separating each degree into an arc minute (‘), and each arc minute into 60 arc seconds (“)
ex. 35 degrees, 27’15”
What does an object’s angular size depend on?
Its actual size and distance
- the farther away an object is, the smaller its angular size will be
What is the horizon?
The boundary between the earth and the sky
What is the zenith?
The point directly overhead (90 degree altitude)
What is the meridian?
An imaginary half circle that stretches from the horizon due south through the zenith to the horizon due north
What is angular size?
The angle of an object appears to span in your field of view
What is angular distance?
The apparent angle that separates two objects in the sky
What does circumpolar mean?
The stars in the north celestial pole remain perpetually above the horizon, circling counter-clockwise around the north celestial pole each day
How does latitude affect which constellation we see?
Because it affects the location of our horizon and zenith relative to the celestial sphere
Does the sky vary with longitude?
No
What does the latitude describe?
It measures the north-south position on earth
What does longitude describe?
The east-west position on earth
Why does the night sky change depending on the time of year?
Because of Earth’s changing position around the Sun
What are seasons caused by?
It is caused by the tilt of the Earth’s axis; the sunlight hits the Earth more directly when the axis is tilted toward the Sun and receives less sunlight when the axis is tilted awa from the sun
What does the June solstice (June 21st) signify?
This is when the Earth’s axis is tipped in the northern hemisphere most towards the Sun
What does the December solstice (December 21st) signify?
This is when the Earth’s axis is tipped in the northern hemisphere farthest from the sun
What is happening at the March and September equinoxes?
The earth’s axis is tilting towards or away from the Sun
Why do the constellations associated with the seasons change over time?
Because of precession
Describe precession?
It is a gradual wobble that alters the orientation of the earth’s axis in space
How long does each cycle of Earth’s precession take?
26,000 years
What is precession caused by?
It is caused by gravity’s effect on a tilted, rotating object
How long does the cycle of lunar phases last?
29.5 days
What is a synchronous rotation?
The phenomenon that describes that the moon rotates on its axis in the same amount of time it takes to orbit earth
What causes a lunar eclipse?
When Earth lies directly between the sun and the moon causing earth’s shadow to fall on the moon
What is a solar eclipse caused by?
When the moon lies directly between the sun and the earth causing the moon’s shadow to fall on earth (or on the sun?)
Why do we only see eclipses when the moon is crossing a node?
Because the moon’s orbit is inclined 5 degrees to the ecliptic plane
What are the conditions for the occurrence an eclipse?
- the phase of the moon is new or full AND
2. the new/full moon occurs when the moon is close to a node
What are the types of lunar eclipses that we can see?
- total lunar eclipse
- partial lunar eclipse
- penumbral lunar eclipse
What phase does the moon have to be in in order to see a lunar eclipse?
A full moon
What are the three types of solar eclipses we can see?
- total solar eclipse
- annular eclipse
- partial solar eclipse
What phase does the moon have to be in in order to see a solar eclipse?
A new moon
How long does the moon take to orbit the earth?
- 3 days relative to the stars (orbital period) or 29.5 days relative to the sun (synodic period)
- the latter happens because each day, the earth is at a different point on the ecliptic
How long is the synodic month?
29 days, 12 hours and 44 minutes
What do we perceive to be the phases of the moon?
The phases of the moon that we observe are the sun’s projection of light during the moon’s orbit around the earth
Generally speaking, when do eclipses occur (i.e. what two things need to happen for us to be able to see an eclipse?)
The eclipses occur when the sun’s light is aligned with a node, and the moon is full or new at that node
How will the moon’s phases look different, depending on north-south positions on earth?
The moon’s phase will appear to be the opposite between the hemispheres
What is an umbra?
when the sun is completely blocked (it is the darkest part of the shadow)
What is a penumbra?
When the sun is partially blocked (it is a lighter shadow)
What is stellar parallax?
the apparent shift in position of a star due to our vantage point in earth’s orbit
How would we be able to perceive the stellar parallax of distant stars?
By speeding up earth’s rotational speed
How do we know which constellations will be visible to us at night?
Based on the sun’s position (ex. if the sun is in Leo, we see Aquarius, because that is the constellation opposite Leo in the celestial sphere)
About how many stars can we se with our naked eye?
About 2000
What is a siderial day?
The point at which the earth completes a 360 degree turn
What is a solar day?
The point at which the sun lies exactly in the same location at 12pm the following day
How many siderial days, and how many solar days are there?
There are 366.26 siderial days and 365.26 solar days
Why does the length of a solar day vary depending on when in the year it is?
- the earth rotates fastest closest to the sun and slowest at the farthest point away
- because of the earth’s axis tilt, the extra rotation needed to get from 12pm-12pm is largest at the solstices and smallest at the equinoxes
Why is the length of a solar day longer in the northern hemisphere compared to the southern hemisphere?
Because the sun is north of the equator for 4 days more than half a year
What does daylight savings time try to accomplish?
It cuts of one end of the rug and sews it back to the front of the rug to make it longer
What are three important consequences of the earth’s precession?
- earth’s tropical year is 20 mins longer than the time from one equinox to the next
- the northern star will change in 15000 years
- the positions of equinoxes shift around orbit
What phase of the moon is it when the moon is directly behind the earth? What bout directly in front of?
Full moon when moon is behind us, new moon when moon is directly in front of us
Name the phases of the moon in their descending order
- full moon
- waning gibbous (D)
- waning third quarter (half)
- waning crescent [)]
- new moon
- waxing crescent [(]
- waxing first quarter (half)
- waxing gibbous (reverse D)
What two things happen once every 18 years and 11 1/3 days? What is this period called?
The perfect coinciding of the moon’s ecliptic, and the occurrence of the full or new moon
- it is called the saros cycle
What is apparent retrograde motion?
When a planet appears to turn westward in relative to the stars
- this is due to Earth passing the planet in it’s orbit
Which planets can we see with the naked eye?
Mercury, Venus, Mars, Jupiter and Saturn
Why did the Greeks assume that the Sun was not at the centre of the solar system?
Because they argued that if this were the case, they would have been able to observe stellar parallax (but they also assumed the planets were much closer)
