Mock Revsion Help Sheet Flashcards
Moon parts
. Sea of crisis . Apennine Mountains . Sea of tranquility . Tyco crater . Kepler crater . Copernicus crater . Ocean of storms
Constellations + pointers
. Cassiopeia
. Cygnus
. Plough- Arcturus and Polaris
. Orion - Sirius, Aldebaran and Pleiades from Orion’s Belt
. Great Square of Pegasus - Fomalhaut and Andromeda
. Southern Cross
. Summer Triangle
Celestial equator
Line on sky above the equator, 0 declination
Ecliptic
Plane on which the Earth orbits the sun, and so the line across the sky upon which the sun appears to move
Zodiacal band
Region of sky a few degrees either side of the ecliptic where the sun and all the planets are always found
Observers meridian
Imaginary line across the sky from the observers southern horizon to the observers northern horizon, also point where objects culminate
Observers zenith
The location directly above the person in the sky, 90 degrees from ever horizon, 90 degrees altitude
Autumnal equinox
Where the ecliptic and the celestial equator meet and the Sun moves from the northern sky to the southern sky (declination 0°)
Winter solstice
Sun has lowest possible declination of -23.5° so the northern hemisphere is tilted directly away from the Sun and it is furthest south from the celestial equator it will get
Vernal equinox
Where the ecliptic and the celestial equator meet and the Sun moves from the southern sky to the northern sky (declination 0°),
Summer solstice
When the Sun has highest possible declination of +23.5° so the northern hemisphere is tilted directly towards the Sun and it is furthest north from the celestial equator it will get.
Tropics
The Tropic of Cancer is L = +23.5° and the Tropic of Capricorn is L = -23.5°. This angle is because that is the tilt of the Earth from 90° away from the ecliptic. Therefore the tropics are the most northerly and southerly latitudes respectively where the Sun can appear on the observer’s zenith (which it does on 21st June for Cancer and 21st December for Capricorn).
What is the Right Ascension of the First Point of Aries, and on which date is the Sun in front of it
FPoA RA = 0 h. The Sun is in front of it at midday on the vernal equinox (about 20th March).
Dave observes that the Sun is due south at his location on a particular date when his watch (set to GMT) says 11:48. The value of the Equation of time for that date is –4 minutes.
What is his AST
1200 – Sun due south means it is on the meridian, culminating, shadows pointing due north, shadows shortest.
MST equation
MST = AST – EoT
Dave observes that the Sun is due south at his location on a particular date when his watch (set to GMT) says 11:48. The value of the Equation of time for that date is –4 minutes.
Calculate MST
MST = AST – EoT = 1200 – (-4) = 1204
Longitude equation
L = ¼(MST – GMT)
Dave observes that the Sun is due south at his location on a particular date when his watch (set to GMT) says 11:48. The value of the Equation of time for that date is –4 minutes.
Calculate Dave’s longitude
L = ¼(MST – GMT) = ¼(1204 – 1148) = ¼(16) = +4°. Positive means E, so 4°E.
What is the radiant of a meteor shower?
The radiant is the point on the sky from which all the meteors seem to come. It is there because as the Earth moves into the region of debris, the first point on the Earth’s atmosphere will be where the rocks fall in from – perspective.
How to name a meteor shower
The names of the meteor showers are from the constellation that is behind the radiant. Eg Perseids has the radiant in front of Perseus.
How to find out where a meteor’s radiant is
- Go out with a star chart or graph paper on a dark, clear night.
- Wait 20 minutes+ to get dark-adapted eyes.
- Watch for a meteor, and put the streak on the star chart (if using graph paper, you’d have to plot a good few stars first!).
- Repeat this process for many meteors.
- Feed the line of each streak backwards until you find a point where all the lines cross.
- Mark on the radiant at this point, and identify the constellation it is in front of to name the meteor shower.
How to prepare for naked eye astronomy
. You must dress warmly (clear sky = cold night)
. Spend at least 20 minutes outside without looking at light sources, so your eyes become dark-adapted (the pupils expand so that more light can enter and you can pick out fainter objects).
. Any light to help should have a red filter on it as the pupil does not respond much to red. The cells at the edges of our retina are more light-sensitive than those in the middle, so looking past/to the side of an object (averted vision) rather than directly at it means it will appear brighter.
Seeing conditions vs weather
Seeing conditions is about the stillness of an object on the sky (whether it is twinkling or not) caused by the amount of movement of air. Weather affects how transparent the sky is (clouds etc).
What is magnitude
Absolute magnitude is the brightness of an object as viewed from a distance 10 pc. Apparent magnitude is the brightness of an object as viewed from Earth. With the naked eye, we can make out apparent magnitudes down to +6 (dimmest). The brightest apparent magnitude in the sky is about 0.
How to determine effect of light pollution
- Research and record the accepted apparent magnitude of some reference stars. Do this for stars with apparent magnitude ranging from 0 to +6.
- Go out and observe these stars and record the apparent magnitude according to your judgement.
- Determine which stars you researched cannot be seen, and write down the apparent magnitude that is not visible.
Why do the phases of the Moon occur?
Phases occur due to the arrangement of the Earth, Sun and Moon. Eg when the Moon is between Earth and Sun, it’s NEW (not visible). When it is on the far side of the Earth from the Sun, it is FULL.
Why do we only see one side of the Moon?
The Moon is TIDALLY LOCKED to the Earth – the heavier side rotates to face us, so the rotational period (spin) of the Moon is exactly the same as the orbital period of the Moon – both 27.3 days.
Lunar libration
Effect of the Moon’s wobble - a slight change in the side facing us so we can see slightly more than 50% of the whole surface – due to the elliptical shape of its orbit and the slight inclination of its orbit (not quite on the ecliptic). The elliptical shape causes it to wobble left and right; the inclination causes it to wobble up and down.
Precession
Wobble of the Earth’s axis over thousands of years. At the moment, Polaris is almost exactly above the north pole (so very close to the celestial north pole) but it will move away before returning in 26000 years’ time.
The altitude of a star of declination +5° for Sheila at latitude 20°N
L = 90 – A + D A = 90 – L + D = 90 – 20 + 5 = 75°
The declination of a star if it is seen with altitude 15° for a Trixie at latitude 0°,
L = 90 – A + D D = L – 90 + A = 0 – 90 + 15 = -75°
The altitude of Polaris for an observer at any latitude
Polaris’ declination is 90°, so the equation becomes: L = 90 – A + 90 so L = 180 – A and A = 180 – L.
That means that whatever your latitude is, IF YOU FACE NORTH INSTEAD OF SOUTH, then Polaris will be that altitude above your northern horizon. Eg: L = 50°N. A = 180 – 50 = 130° above southern horizon. That’s the same as 50° above the northern horizon.
Observer latitude
Observer Latitude = 90 – object Altitude at culmination + object Declination
L = 90 – A + D
Umbral contacts
U1 is the 1st umbral contact, U2 is the 2nd umbral contact, U3 is the 3rd umbral contact, U4 is the 4th umbral contact.
• Measure the time from 1st to 2nd umbral contact (U1 to U2).
• Measure the time from 1st to 3rd umbral contact (U1 to U3).
• Do the second time (time to cross the umbra) divided by the first time (time to enter the umbra). The answer is the number of times the Earth’s diameter is bigger than the Moon’s.
Local Sidereal Time and Local Hour Angle
The local sidereal time (LST) is the value of right ascension that is due south for the observer (the RA on the local meridian).
The local hour angle is the angle away from the meridian an object appears (H).
The right ascension of an object is the angle it is away from the First point of Aries (vernal equinox point), α.
So the right ascension of a star can be calculated by doing: RA (α) = LST – LHA (H).
