Earth to Moon distance: 250,000 miles or 380,000 kilometers
Diameter of the Moon: 3,500 kilometers
Shape of planet Earth:
Earth is not a sphere but an oblate spheroid as it is actually slightly flattened at it's poles (by 42km).
Diameter of Earth: 13,000 km (rounded off)
Time it takes Earth
to rotate 1 degree: 4 minutes
Sidereal day:
23 hours 56 minutes. A sidereal day is the length of time it takes a planet to rotate from the perspective of a distant star which for Earth is 23 hours 56 minutes and 4 seconds.
23 hours 56 minutes. A sidereal day is the length of time it takes a planet to rotate from the perspective of a distant star which for Earth is 23 hours 56 minutes and 4 seconds.
Solar day:
24 hours. A solar day is the length of time it takes the Sun to reach the same point in our sky from one day to the next which is 24 hours.
The discrepancy between a sidereal and a solar day is due to the earth's orbit around the sun. During the time it takes Earth to rotate once on it's axis (a sidereal day) it has also moved in it's orbit around the sun and a further 4 minutes are needed to bring the Sun back to the same point in the sky (a solar day).
This also shows why the stars appear to rise 4 minutes earlier each day.
Star Trails:
Star trails can be used to measure the length of time it takes Earth to rotate once on it's axis.
If you measure the angle of a star trail using a protractor and for instance it measures 30 degrees and the star trail image was a 2 hour exposure then 30 degrees is 1/12th of 360 degrees ( a full circle/sphere) so 12 x 2 hours = 24 hours.
Units of measurement:
Astronomical unit (A.U) is the unit we use to measure distances within our solar system. The average distance between Earth and the Sun is 1 astronomical unit.
Parsec is a unit we use to measure the distance to objects outside of our solar system. 1 parsec is about 3.26 light-years.
If measuring the size of Galaxies we use kiloparsecs (kilo = thousand) and when measuring distances between galaxies we normally use megaparsecs (mega = million).
Definition of a parsec:
One parsec is the distance at which a star would have a parallax angle of one arc second.
The Moons orbit:
It takes 27.3 days for the Moon to make a complete orbit around the Earth and 29.5 days for the lunar phases to go full circle from new to new Moon. The 2.2 days difference is due to Earth having moved quite a distance around the sun during the time since the previous new Moon.
Phases of the Moon:
Map of the Moon:
Man first stepped on to the surface of the Moon in July 1969. The Apollo 11 mission was followed up with a further 5 missions between 1969 - 1972.
As well as collecting rock samples Astronauts from each mission installed and activated scientific experiments which provided data for years afterwards. These instruments where generally known as ALSEP (apollo lunar surface experiments package)
Purposes of the instruments in the ALSEP where to detect, monitor and analyse :
. The speed and intensity of the solar wind
. Presence of micro-meteorites
. Composition and pressure of the lunar atmosphere
. Structure of the Moon's interior
. Thermal and electrical properties of the Lunar sub surface
. Lunar dust
. Minute changes in Lunar gravity
. Measure Earth-Moon distance
Positions of Earth, Moon
and Sun during a solar eclipse:
Positions of Earth, Moon
and Sun during a lunar eclipse:
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| The Sun in white light G.Marshall |
Distance from the Sun
to the Earth: 150 million km
Temperature of the
Sun's photosphere: 5800 k
Temperature of
the corona: 2 million k
Solar System
Since objects in our solar system where re-classified in 2006 we now have 8 planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
There are also many dwarf planets in our solar system including Ceres and Pluto.
using a telescope:
Uranus - It was discovered in 1781 by William Herschel.
Fist planet located photographically:
Pluto - Discovered 1930 by Clyde Tombaugh (A planet at the time. Now re-classified a dwarf planet.)
Closest dwarf planet
to the sun :
Ceres - It lies between Mars and Jupiter.
Other bodies in the Solar System include:
. Satellites/Moons - Which are in orbit around planets. We only have one orbiting Earth but some planets have over 60 known Moons!
. Asteroids - Large chunks of mostly rock which are not big enough to form a spherical shape and most of which are in orbit in the asteroid belt between the orbits of Mars and Jupiter.
. Comets - Consisting of rock, ice and dust they where formed and are generally located in the outer solar system.
Short period comets are thought to originate from the Kuiper belt
Long period Comets are thought to originate from the Oort cloud.
. Centaurs- Objects which resemble both Asteroids and Comets and are in orbit of the sun between Jupiter and Neptune.
. Trans- Neptunian objects - Objects orbiting the Sun beyond the orbit of Neptune
As Saturn's are the only planetary ring system easily visible to us here on Earth many people think it is the only planet with them but all four of the gas giants have ring systems of some description,
Kepler's law
Kepler's first law states that all planets move in elliptical orbits around the Sun with the sun at the focus of each ellipse.
Keper's second law relates a planets speed to it's distance from the Sun. An imaginary line from a planet to the Sun sweeps out equal areas in equal amounts of time. The planet being much closer at one point in it's orbit travels faster than when it is at it's furthest so that it has traveled a further distance but swept out an equal area in a equal amount of time.
Kepler's third law - At GCSE this has been simplified slightly but we use an equation relating the orbital period of a planet in years (T) to it's mean orbital radius in astronomical units (R).
Simply a planets orbital period squared is equal to it's mean orbital radius cubed.
T2 = R3
If the value for either orbital period in years (T) or mean orbital radius (R) are given it is simple to square T and then cube root of the answer or vice versa cube R and then square root of the answer using a calculator to give the value for the other.
So if a planet has a orbital period (T) of 12 years it's mean orbital radius (R) would be....?
12 squared = 144 cube root of 144 = 5.2414..
R = 5.2 AU
Exoplanets
Exoplanets are planets in orbit around stars other than the Sun.
Many planets and planetary systems have now been detected around other stars and the number is increasing rapidly as technology improves.
The three main techniques currently used to detect Exoplanets are:
. Transit method - Looking for minute dips in a stars brightness indicating the transit of a planet
. Astrometry - Precisely measuring the position of a star. A massive exoplanet due to it's large gravitational pull can cause a star to noticeably move or wobble slightly as it orbits. These small wobbles we detect and can infer the existence of the planet from.
. Radial Velocity - The wobbling of a star as described in the astrometry method also causes a shift in wavelength. The red-shift or blue-shift of it's spectral lines can be detected with spectroscopy.
Stars
A stars brightness is measured in magnitudes. A difference of 5 magnitude corresponds to a star being 100 times brighter than another. So a magnitude difference of 1 is equal to the fifth root of 100 which is 2.511886432 or just 2.5 for the purposes of GCSE (Thankfully!)
Magnitude difference Brightness ratio
1 2.5
2 6.25
3 16
4 40
5 100
6 250
The apparent magnitude (m) of a star depends on a few factors:
. Total energy radiated by a star / star's size & temperature
. Distance to the star
. The amount of interstellar gas and dust.
. Amount of light absorbed by Earth's atmosphere
A star which is actually brighter may appear dimmer as it is further away and vice versa.
The intensity of light and other forms of radiation from a star obey the inverse square law which means if the distance doubles then the intensity of light is (2 squared) 4 times dimmer, if it was 4 times further then 4 squared is 16 times dimmer...and so on.(remember a brightness ratio of 16 is equivalent to 3 magnitudes)
The inverse square law is also obeyed by gravity, if the distance doubles the force of gravity is one quarter (the inverse of 2squared) of it's previous value. If the distance is 5 times greater the force is 1/25 of it's previous value (the inverse of 5 squared).
The absolute magnitude (M) of a star is defined as the apparent magnitude the star would have if observed from a standard distance of 10 parsecs
Both the absolute (M) or apparent (m) magnitude of a star can be worked out given that you know one of the values and it's distance in parsecs using this fomula:
M = m + 5 - 5 log d
Where d is the distance in parsecs. Logarithms are beyond GCSE astronomy and this formula will be used to calculate M or m and not d.
To simplify distances are given in multiples of 10
log 10 = 1
log 100 = 2
log 1000 = 3
....
log 100 000 = 6 etc..
So if a star has an apparent magnitude of 4.6 and is at a distance of 100 parsecs it's absolute magnitude would be...
M = m+5 - 5 logd M= 4.6 + 5 - 5x2 M= 9.6 - 10
M = - 0.4
To work out apparent magnitude (m) you simply reverse the formula:
m = M - 5 + 5 log d
GCSE Astronomy past papers <<< Direct link
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