A6 - Celestial observation

Question 1

How could you tell the difference between a satellite and a plane?

Aside from its brightness, shape/size, and colour

Answer 1

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

Question 2

What is the difference between an asterism and a constellation?

Answer 2

-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)

Question 3

Draw Cassiopeia:

Answer 3

α is Schedar

Question 4

Draw Cygnus:

Answer 4

Question 5

Draw Orion:

Answer 5

Question 6

Draw the Plough:

Answer 6

continue the Polaris line and you get to Cassiopeia

Question 7

Draw the Southern Cross:

Answer 7

Question 8

Draw the Summer Triangle:

Answer 8

contains Deneb, from Cygnus

Question 9

Draw the Square of Pegasus:

Answer 9

Question 10

List all 7 constellations/asterisms you need to know:

Answer 10

-Cassiopeia
-Cygnus
-Orion
-Plough
-Southern Cross (Crux)
-Summer Triangle
-Square of Pegasus (Great Square)

CCOP, SSS

Question 11

How do you find Polaris and Arcturus from the Plough?

Answer 11

-use line between Dubhe and Merak and go up 3 times further to find Polaris

-follow curve of the “handle” to find Arcturus

Question 12

How can you use Orion’s Belt to find Sirius, Aldebaran, and the Pleiades?

Answer 12

-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

Question 13

How can you find Fomalhaut from the Square of Pegasus?

Answer 13

-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

Question 14

How can you find the Andromeda Galaxy from the Square of Pegasus?

Answer 14

-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

Question 15

Why are there different names for constellations, asterisms, and stars?

Answer 15

-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

Question 16

Describe what a star chart is:

Answer 16

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

Question 17

Describe what a planisphere is:

Answer 17

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

Question 18

Give some pros and cons of using planispheres:

Answer 18

-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

Question 19

What is the celestial sphere, poles, and equator?

Answer 19

-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

Question 20

Name and describe the 2 celestial coordinate systems:

Answer 20

equatorial - declination and right ascension

horizonal - altitude and azumith

Question 21

How does the equatorial coordinate system work?

Answer 21

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)

Question 22

How does the horizonal coordinate system work?

Answer 22

altitude - angle measured up from the horizon (also known as elevation)

azumith - angle measured from North going Eastward

Question 23

Name a disadvantage of using the horizonal coordinate system:

Answer 23

-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

Question 24

What is the celestial meridian?

Answer 24

a great circle connecting the NCP with the SCP, passing through the zenith and nadir

Question 25

How do you find the maximum altitude of a star, given your latitude and its declination?

Answer 25

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

Question 26

How would you find your latitude using a celestial method? Explain how this method works:

Answer 26

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

Question 27

Why can’t you use Polaris to determine your longitude?

Answer 27

its position is the same at different longitudes

Question 28

What is the hour angle? What does it tell you about an object?

Answer 28

-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)

Question 29

What is local sidereal time?

Answer 29

-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 ♈

Question 30

What is the relationship between a culminating object and its right ascension? State the relevant equation that explains this:

Answer 30

-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.

Question 31

What does it mean when a star culminates?

Answer 31

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

Question 32

What does it mean by diurnal motion?

Answer 32

the apparent motion of the sky rotating from an observer’s point of view

Question 33

What is a circumpolar star? How can you tell if a star is circumpolar given its declination and the observer’s latitude?

Answer 33

-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

Question 34

At what 2 points does an object cross the meridian?

Answer 34

upper transit - highest point in sky (ie culmination)

lower transit - lowest point in sky

Question 35

What factors affect the visibility of stars?

Answer 35

-rising/setting times (for non-circumpolar stars, try halfway between their rise/set times)
-seeing conditions (light pollution, atmospheric turbulence)
-weather conditions
-landscape

Question 36

Name and describe 2 techniques for naked-eye observations:

Answer 36

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

Question 37

How could you investigate the effect of light pollution on star observations?

Answer 37

-identify reference stars with a range of known magnitudes
-compare which of these stars can/can’t be observed