P7- Section 1

Question	Answer
What is a solar day?	The time that passes between the sun appearing at its highest point in the sky on two successive days.
Describe the apparent motion of the moon and stars?	The earth's daily rotation makes the moon appear to travel across the sky from east to west- but the Moon is also orbiting the Earth. This orbital motion means the Moon takes longer than 24 hours to return to the same position in the sky. On average the Moon rises about 50 minutes later each day than the previous day. The stars also appear to move across the sky. The stars take a little under 24 hours to return to the same position in the night sky.
What is a sidereal day?	The reason it takes slightly less time for a star to be seen in the same position is because it takes 23 hours 56 minutes for the Earth to rotate through 360 degrees. This is known as a sidereal day. It is different from a solar day because of the Earth's orbit around the sun. By the time the Earth has turned once it has moved around 1 degree relative Sun. This means it takes a further 4 minutes for the sun to be in the same position.
Explain the phases of the moon.	We see the moon because it reflects the Sun's light. As it moves it reflects more, or less, light towards the Earth. We see the sunlit part of the Moon change shape from a full circle to a crescent and back again.These shapes are known as the phases of the moon.
What is a lunar month?	It takes 27.3 days for the moon to orbit the Earth but because the Earth is moving around the Sun, it takes the Moon a bit longer to get back to the same position relative to the Sun. In fact there are 29.5 days between two full moons. This is a lunar month.
Explain the phases of the moon with a diagram.	Image: photo2 (image/jpg)
Why do the Sun and the Moon appear to be the same size from the earth?	The Sun's diameter is about 400 times bigger than that of the moon but it is also 400 times further away.
Explain both types of eclipses?	As the moon orbits, it sometimes passes between the Sun and the Earth, casting a shadow on the Earth. This is called a solar eclipse. When the Moon travels into the Earth's shadow it can no longer reflect the Sun's light. This is called a lunar eclipse.
Draw a diagram of a solar eclipse.	Image: photo6 (image/jpg)
Draw a diagram of a lunar eclipse.	Image: photo_5 (image/jpg)
Why is there not a lunar eclipse every month?	The orbit of the moon is not in the same place as the Earth's orbit around the Sun. We only get an eclipse when the Sun, Moon and Earth all line up. That happens only two or three times a year on average.
What planets can be seen with the naked eye? What does the amount of light that a planet reflects depend on?	Mercury, Venus, Mars, Jupiter and Saturn. How close the planet is to the Sun; how close it is to the Earth; the size of the planet; the nature of the planet's surface
What is retrograde motion and why did it change the way we viewed the solar system?	If you trace the path of a planet at the same time every night it takes an unusual path, sometimes appearing to go backwards. This is called retrograde motion and it couldn't be explained using a model of the solar system which centred around Earth. Although this model could explain the motion of the Sun, Moon and stars it couldn't explain the motion of planets, nor the way they varied in brightness. Scientists realised that the motion of planets would make sense if all planets revolved around the Sun. The planets close to the Sun move faster in their orbits than the outer planets do. So Earth catches up with planets outside it then goes past them. This makes it look like they are going backwards.
Use a diagram to show retrograde motion.	Image: photo (image/jpg)
Why does the sky look different in summer and winter?	Because the Earth has moved to the other side of its orbit around the Sun. The winter sky may seem more impressive because the nights are long and there are some very bright stars but in the summer we can see more stars as our skies aim towards the centre of the galaxy. The hazy look to a clear summer sky is the light from billions of stars in the central disc of our galaxy.
How do we locate stars?	With two pieces of information: which direction to face and how high to look. The height of the star above the equation is given as an angle called the declination. A negative declination means that the star is south of the equator. The angle that defines the direction (east-west) is called the right ascension. These two angles form a coordinate system for locating or describing the location of a star.
What is the celestial sphere? What is it used for?	An imaginary invisible dome stretching around the earth. The light from every astronomical object is projected onto this dome. The celestial sphere has a north pole directly over the Earth's North Pole and an equator over the Earth's equator. We use the celestial sphere as a reference for the coordinate system.
What are the declination and right ascension measured in?	The declination is measured from the celestial equator in degrees, minutes and seconds. One minute (1') is equal to 1/60 degree. One second (1'') is equal to 1/60 of a minute. Although it is an angle, the right ascension is measured in units of time: hours, minutes and seconds. The celestial sphere rotates with the earth once every 24 hours, so 24 hours is equal to 360 degrees. So one hour is equal to 15 degrees.
What is the ecliptic and what does it show?	The plane of the Earth's orbit. Viewed from the Earth the ecliptic shows the path of the Sun during the year. The right ascension is measured from the point where the Sun moves to the northern hemisphere. This is the spring (or vernal) equinox and it happens around 21 March every year.
Show this in a diagram.	Image: photo (image/jpg)
What is refraction?	The speed of a wave depends on the medium which it travels. As the waves slow down they get closer together so the wavelength decreases. This is called refraction. Refraction does not change the frequency of the waves.
What happens when a wave hits the boundary between two materials at an angle?	Refraction causes it to change direction. When travelling into a medium where the speed is lower, it is like a car driving into mud. The wheel that hits the mud first must slow down causing it to change direction. If light passes from air into a glass block, it will change direction as it enters the block and again when it leaves. This is because light travels more slowly in glass, or other transparent medium, than in air.
Draw a diagram to show light entering and leaving a glass block.	Image: photo (image/jpg)
Explain the parts of the diagram.	The line represents a light ray and the arrow represents the direction it is moving in. The normal is a line drawn at a right angle to the block. Refraction causes light to deviate from the normal as it enters the block and away from the normal as it leaves the block.
How can we bring light to focus?	By shaping the glass block we can use refraction to bring light to a focus. A glass convex lens, which is thicker in the middle than at the edges, can refract light rays so that they come together to converge. Rays of light that strike the lens parallel to its axis will meet at the focal point or the focus.
Show this in a diagram.	Image: photo (image/jpg)
Explain focal length and power for convex lenses.	Convex lenses bring light together. Parallel light rays are brought together at the focal point of the lens. The distance from the lens to the focal point is called the focal length. A lens is said to be more power if it can converge the light in a shorter distance. So a powerful lens has a short focal length. The power of a lens is measured in dioptres and is calculated using this equation: 1/ focal length (metres)
Explain the light from stars?	Because stars are so far away from the earth by the time the rays reach it they are virtually parallel. They are therefore brought to a focus at a single point. If the incoming light is not parallel to the principal axis- the imaginary line that goes through the centre of the lense- the focus will be away from the axis.
Show an image formed below the principal axis.	Image: photo (image/jpg)
What are objects that look larger than a point called?	Extended objects
How do we draw ray diagrams of extended objects?	We represent the object by an arrow and then follow what happens to rays of light leaving the tip of the arrow. We represent the lens by a vertical line. Although light leaves the arrow tip in every direction, there are three rays we can predict accurately: 1) Any ray that hits the lens parallel to the axis will be refracted through the focal point. 2) A ray that travels through the focal point will be refracted parallel to the axis (reverse of ray 1) 3) A ray that hits the centre of the lens will be undeviated. The focusing point of the arrow tip will be formed where the rays meet again.
Draw a diagram to show this.	Image: photo__3_.JPG (image/JPG)
What is a simple telescope made up from?	Two convex lenses: the objective lens and the eyepiece. The objective lens has a long focal length (low power) and the eyepiece has a short focal length (high power). The lenses are placed so that the length of the telescope is the sum of their focal lengths.
Explain the eyepiece and objective lens in more detail	Objective lens has a larger diameter than the eyepiece. The objective needs to have a large area to collect more light to make a brighter image. The objective forms an image of the object at the focal point. The eyepiece is smaller than the objective, but has a greater power. The eyepiece is used like a magnifying glass to look at the image formed by the objective. Your eye sees a magnified image.
What happens when you look at a distant object?	Your eye refracts the parallel rays of light to form an image on your retina at the back of your eye. We say your eye is focused on infinity as the source of the parallel rays if effectivly infinity.
What happens when we look through a telescope? Explain magnification.	The effect of the telescope is to increase the angle between the light rays and the axis. This increases the apparent separation between stars and makes extended objects, such as the Sun, Moon and planets, look bigger. If the image of an extended object through the telescope appears twice as long as with the naked eye, we say the telescope has a magnification of 2. The magnification is equal to the ratio of the angle of light entering the telescope relative to the principle axis compared to that angle when the light is going through the eyepiece.
Draw a diagram to summarise general simple telescopes.	Image: photo__3_.JPG (image/JPG)

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