A6 - Celestial observation Flashcards
How could you tell the difference between a satellite and a plane?
Aside from its brightness, shape/size, and colour
satellites reflect light from the Sun, and would disappear before reaching the horizon, because they would go into Earth’s shadow and no longer reflect any more light
What is the difference between an asterism and a constellation?
-an asterism is just a popular pattern of stars (eg Orion’s Belt, Plough)
-a constellation is an official group of stars and the area surrounding it (eg Orion, Ursa Major)
Draw Cassiopeia:
α is Schedar
Draw Cygnus:
Draw Orion:
Draw the Plough:
continue the Polaris line and you get to Cassiopeia
Draw the Southern Cross:
Draw the Summer Triangle:
contains Deneb, from Cygnus
Draw the Square of Pegasus:
List all 7 constellations/asterisms you need to know:
-Cassiopeia
-Cygnus
-Orion
-Plough
-Southern Cross (Crux)
-Summer Triangle
-Square of Pegasus (Great Square)
CCOP, SSS
How do you find Polaris and Arcturus from the Plough?
-use line between Dubhe and Merak and go up 3 times further to find Polaris
-follow curve of the “handle” to find Arcturus
How can you use Orion’s Belt to find Sirius, Aldebaran, and the Pleiades?
-follow Belt in the opposite direction to his shield to get to Sirius
-go towards his shield and it will go past Aldebaran and then the Pleiades
How can you find Fomalhaut from the Square of Pegasus?
-take the slightly reddish star Scheat and use the star Markab (clockwise (NH) around the square from it)
-use the line that they form and follow it (quite far) to find Fomalhaut
Fomalhaut just barely gets above the horizon
How can you find the Andromeda Galaxy from the Square of Pegasus?
-take slightly reddish star Scheat and go to Alpheratz (anticlockwise (NH) around square from it)
-follow the curve 2 stars past the square and find Mirach
-from Mirach, go past 2 stars perpendicular to the original path and find the Andromeda Galaxy
Why are there different names for constellations, asterisms, and stars?
-different cultures in other countries developed different names for them (eg Big Dipper in USA, Plough in UK)
-they needed a way of remembering them, so they gave them culturally identifiable names so that they could refer to them more easily
Describe what a star chart is:
a map of the sky that shows the stars that are visible in a particular season
They can have 2 maps, where one shows the southern sky and the northern sky at a particular location
Describe what a planisphere is:
2 discs that can rotate and show you what stars are visible from a particular latitude at a given time, where the axis of rotation is fixed at the NCP
Give some pros and cons of using planispheres:
-can determine what celestial objects are above the horizon on a given date/time, as it shows the horizon
-can determine the rising/setting/culmination times for celestial objects
-can be adjusted to the time/month of the observation
-only designed for a specific latitude, whereas star charts can be used anywhere on Earth
What is the celestial sphere, poles, and equator?
-imaginary sphere that older cultures believed the stars would all sit on, at a certain distance from Earth
-celestial poles are the points directly above the North/South geographical poles
-celestial equator is the line that is always directly above the geographical equator
Name and describe the 2 celestial coordinate systems:
equatorial - declination and right ascension
horizonal - altitude and azumith
How does the equatorial coordinate system work?
declination - angle measured from celestial equator, increases towards NCP
right ascension - angle measured from First Point of Aries, increases going anticlockwise as viewed from above (same as longitude)
How does the horizonal coordinate system work?
altitude - angle measured up from the horizon (also known as elevation)
azumith - angle measured from North going Eastward
Name a disadvantage of using the horizonal coordinate system:
-horizonal system is fixed relative to the Earth, not to the stars
-this means that the star’s coordinates change over time, and are also different if you change your position on the Earth
What is the celestial meridian?
a great circle connecting the NCP with the SCP, passing through the zenith and nadir
How do you find the maximum altitude of a star, given your latitude and its declination?
90-L+d, or 90+L-d (whichever comes out to be <90°)
always plug in the declination with its sign for both equations
You can also do it geometrically like in the diagram
How would you find your latitude using a celestial method? Explain how this method works:
your latitude is equal to the altitude of the NCP (use Polaris because it’s close enough)
You should know how to draw the diagram below
Why can’t you use Polaris to determine your longitude?
its position is the same at different longitudes
What is the hour angle? What does it tell you about an object?
-the angle (usually given as a time) between an object and the local meridian, with respect to the celestial pole
-the hour angle tells you how long it has been since an object culminated (if it is negative then it hasn’t culminated yet)
What is local sidereal time?
-the hour angle of the First Point of Aries
-imagine the sky as a big 24 hour clock that has the entire face rotating anticlockwise (if you face North), and the LST is just the right ascension of the object that is culminating
In this image, the labeled angle indicates the LST, which is the HA of ♈
What is the relationship between a culminating object and its right ascension? State the relevant equation that explains this:
-if an object is culminating (HA = 0), its right ascension is equal to the local sidereal time
Note that the RA angle goes clockwise - in a vector sense, this doesn’t make logical sense and wouldn’t add up to LST, but the actual value for RA going clockwise is positive, so the maths works out.
What does it mean when a star culminates?
it reaches the highest point in the sky from an observer’s point of view (always lies on the celestial meridian)
And its right ascension is equal to the local sidereal time
What does it mean by diurnal motion?
the apparent motion of the sky rotating from an observer’s point of view
What is a circumpolar star? How can you tell if a star is circumpolar given its declination and the observer’s latitude?
-a star that never goes below the horizon (ie negative altitude) from an observer’s point of view as the Earth rotates every 24hrs
-if 90-d <= latitude, then the star is circumpolar
At what 2 points does an object cross the meridian?
upper transit - highest point in sky (ie culmination)
lower transit - lowest point in sky
What factors affect the visibility of stars?
-rising/setting times (for non-circumpolar stars, try halfway between their rise/set times)
-seeing conditions (light pollution, atmospheric turbulence)
-weather conditions
-landscape
Name and describe 2 techniques for naked-eye observations:
dark adaptation - wait a sufficient amount of time in dark conditions away from any bright light sources
averted vision - study an object whilst placing it at the edge of your field of view
Your more light-sensitive rods (away from fovea) can pick up the fainter light
How could you investigate the effect of light pollution on star observations?
-identify reference stars with a range of known magnitudes
-compare which of these stars can/can’t be observed
