1. The night sky and the Sun-Earth-Moon system Flashcards
What is the difference between a sidereal day and solar day?
Sidereal day represents Earth’s rotation with respect to stars, and is 4 minutes shorter than a solar day, which is measured in respect to Sun’s position in the sky.
Imagine a hypothetical planet whose axis of rotation was perpendicular to the plane of its orbit. Describe the passage of day and night and the passage of seasons on such a planet.
Day and night will always be equal! Seasons will be totally determined by eccentricity of its orbit, meaning how circular or not the orbit is. Summer will be when the planet is closest to the star, and winter when it is farthest from its star. Both the northern and southern hemispheres, in fact all points of the planet, will experience the same seasons!
How would the path of things in the sky change over night and over a year if the Earth spun in the reverse direction but continued to orbit as normal? What about if the Earth didn’t spin at all but continued to orbit as normal?
Nightly motions are determined by our rotation, yearly motions are determined by our orbit.
Spinning in reverse would make things move backward over the course of a night or a day, so everything would rise in the west and set in the east - basically everything would be moving backwards in the sky. If we didn’t rotate on our axis at all then there would be no daily motion of objects, and everything would only move due to the orbit. Our days would be half a year long!
Just as the Moon has phases, so too does the Earth. Imagine an astronaut standing on the surface of the Moon looking at the Earth. Describe the phases of the Earth that the astronaut would see. What would this astronaut see the Earth’s phase to be when we see the Moon as a crescent, or when we see a new Moon?
Essentially if we view a diagram of the phases of the Moon, we can look at the Moon phase opposite to what we are asked about: So when we see the Moon as a crescent the Earth is gibbous. When the Moon is new the Earth is full.
We all know the Moon gives us some cool solar eclipses, so now let’s think about that a bit. Imagine the Moon were to become suddenly larger than its current size, how would this change the way solar eclipses appear? What if the Moon suddenly shrank in size? Is there another way to achieve the same changes? If so, what other action would mimic the Moon increasing in size?
The Moon has the same angular size as the Sun, where its angular size depends on its actual size and its distance.
Bigger Moon = more eclipses.
Smaller Moon = less eclipses and zero total eclipses.
The same thing can be achieved by moving the Moon closer (to make it look bigger) or farther away (to make it seem smaller).
What path does the Sun appear to take across the sky?
The Ecliptic
What is the Zenith, Horizon & Meridian?
Zenith = top of celestial sphere, 90* from the horizon
Horizon = where the dome/celestial sphere meets the Earth
Meridian = arc that goes through the north point of the horizon, zenith, and south point of the horizon
How do stars move through the local sky? (depends on your latitude and the stars position in the sky)
North & south celestial pole = the stars would all circle around the celestial pole with none rising or setting
Equator = all stars rise and set. They move up from the east side of your horizon and set down on the west side of your horizon
What is a light year?
The distance light travels in one year
c=3x10^8 m/s
Why do stars rise and set?
Rotation of planet Earth
How did the invention of the telescope change our view of the cosmos?
Galileo use an early telescope to prove the Copernican theory. He found Venus goes through phases like the moon, showing that it must revolve around the Sun.
Why is the Moon important?
Help stabilise our planets wobble and moderate our climate
What does the universe look like from Earth?
A celestial sphere
How do we know the Earth is round and how big?
Object sink below the horizon, lunar eclipses, different stars visible as you move north and south.
Eratosthenes measured angular displacement of the Sun from overhead at Alexandria and Syene (5,000 stadia away) at the same time. He found the angular displacement was 7.2. There are 360 in a circle, making 7.2* = 1/50 of a circle. 5,000x50 = 250,000 stadia, roughly 40,000km.
What object has the fastest apparent motion across the sky?
Moon
