Skys Laboratory

8/9/2019 Skys Laboratory

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2 Lab 3 The Sky

PARTIFINDING THEPOLE STAR

a) If this is an evening lab and if the sky is clear go outside and try to find the Big Dipper and Polaris.

b) Here is cartoon drawing of a part of the sky — without the constellations lines drawn in. After all,that’s what the sky looks like! Bright stars are represented by big dots, and fainter stars by smallerones. In the pattern below find the Big Dipper, Cassiopeia, and Polaris. Play the game of connect-the-dots , and identify the remaining constellations.


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The Sky #1 Lab 3 3

Zenith of the person on the Globein the Celestial Sphere.

Points to the top of the ceiling.

PARTIITHE CELESTIAL SPHERE

Have you noticed that the night sky appears to be different if you travel much further north, or south?You still see the same constellations, but they will appear to be at a different location on the night sky.Also, if you follow the stars motions throughout the night, the stars will follow different paths. In thisexercise you will learn to how your location on Earth affects where you see Polaris. You will also learnto visualize the motions of the stars on the night sky.

1) Orienting yourself

Imagine you are standing somewhere on Earth (let’s take Boston), and you are looking up. The pointdirectly above you is the Zenith. Perpendicular to the Zenith, all the way around you is the Horizon.As illustrated in the Figure below, position the Celestial Sphere so that the Zenith of a miniature

person standing on the Globe in Boston (see below) point to the same Zenith as you standing in theclassroom (turn the Celestial Sphere until Boston is at the top and pointing to the ceiling).

The proportions in this Figure are “a little off”.

You in theastronomy lab –

looking at thecelestial sphere in

front of you.

YourZenith(Top ofCeiling)


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4 Lab 3 The Sky

2) Identifying where is what on the CELESTIAL SPHERE

Grab on of the Celestial Spheres provided by your instructor. Before you start filling out the Tableon the next page make sure you can locate the following on the Celestial Sphere in front of you.

a) On that CELESTIAL SPHERE where is

The Z ENITH and the H ORIZON The Earth’s E QUATOR and the C ELESTIAL E QUATOR The NORTH & SOUTH CELESTIAL POLES

b) Which numbers on the CELESTIAL SPHERE are telling you:Your LONGITUDE and LATITUDE The RIGHT ASCENSION and DECLINATION of a stat in your Z ENITH What is the ALTITUDE of a Star in your Z ENITH (show the angle)

c) Chose any star on the CELESTIAL SPHERE , spin the sphere and watch the path of that star. Next,imagine yourself standing on the Globe inside the CELESTIAL SPHERE – watch how the star rises

above the horizon, reaches its highest, and then sets below the horizon. The path of the starshould look somewhat similar to the path of the star in Figure 4 of the Pre-Lab.

3) Correlating various Angles

Imagine you are standing on the North Pole, then in a city at a latitude of 60, at 30 degrees andfinally at the equator. Identify specific angles and compare these angles to other angles on theCelestial Sphere. Complete the table on the next page. Below is an explanation of the meaning ofevery column of that table.

Column A) Take the CELESTIAL SPHERE and orient it in such a fashion that the E ARTH ’S NORTH P OLE points towards the ceiling. Now imagine you’re standing on the N ORTH P OLE ― (on thesmall E ARTH ) and you’re looking straight up.

Column B) Your LATITUDE .

Column C) Locate P OLARIS . What is the ALTITUDE of Polaris? Write this into the table.

Column D) Identify the Z ENITH and the H ORIZON on the Celestial Sphere in front of you. Identify astar in your Z ENITH . What is the D ECLINATION of that star?

Column E) Determine the angle between the star in your Z ENITH and P OLARIS ?

Column F) Determine the angle between your H ORIZON and the C ELESTIAL E QUATOR .

Column G) Compare the angle between the star in your Z ENITH and P OLARIS to the angle betweenyour H ORIZON and the C ELESTIAL E QUATOR . Determine the difference between theseangles.

Column H) Compare the angle between your H ORIZON and the C ELESTIAL E QUATOR to yourLATITUDE . How are the two angles related? Add both angles and write into the table.

Column I) Compare the ALTITUDE of P OLARIS to your LATITUDE . Determine the difference betweenthese angles and write that into the table.


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A

B

C

D

E

F

G

H

I

Y o u r

l o c a t i o n

Y o u r L A T I T U

D E

A L T I T U D E o f

P O L A R I S

D E C L I N A T I O N

o f

a s t a r i n Z E N I T H

A N G L E b e t w e e n

P O L A R I S &

Z E N I T H

A N G L E b e t w e e n

H O R I Z O N &

C E L E S T I A L

E Q U A T O R

D I F F E R E N C E

b e t w e e n

A N G L E C e l E q – H o r

a n d

A N G L E Z e n - N

S U M o f

A N G L E H o r – C e l E q

a n d L A T I T U D E

D I F F E R E N C E

b e t w e e n

A L T I T U D E o f

P O L A R I S —

L A T I T U D E

S y m b o l

L A T

A L T N

d e c

A N G L E Z e n - N

A N G L E C e l E q – H o r


– A N G L E Z e n – N


+ L A T

A L T N

–

L A T

N o r t h P o l e

9 0 o

O s l o

6 0 o

W a s h i n g t o n

3 0 o

E q u a t o r

0 o

B o s t o n

C o m m e n t o n t h i s t a b l e . W h a t c o r r e l a t i o n d o y o u f i n d i n g e n e r a l ? A r e s o m e o f t h e c o l u m n s a l w a y s t h e s a m e ? W h a t i s t h e c o r r e l a t i o n

b e t w e e n y o u r L A T I T U D E a n d

t h e A L T I T U D E o f P O L A R I S ?


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6 Lab 3 The Sky

PARTIII:THE APPARENT MOTION OFSTARS IN THESKY

With the aid of the celestial sphere figure out what the night sky looks like, where the North Pole isrelative to your location on Earth, and how the stars appear to move throughout the sky. Use theCelestial Sphere as in Part II, but rotate the Sphere around its axis. This will show you the motion of thestars; where they rise, where they transit (when they are at their highest point), and where they set.

For EACH of the four diagrams below, draw the following:a) Using a black pen label the H ORIZON , and N, S, E, W on the H ORIZON

b) Using a blue pen label P OLARIS , the N-S AXIS , the C ELESTIAL E QUATOR c) Using a red pen label your L ATITUDE and the A LTITUDE of P OLARIS d) Using a green pen draw the paths of the two stars in ALL FOUR diagrams

Zenith Zenith

North Pole / 90° Oslo / 60°

Zenith Zenith

Washington / 30° Equator / 0°


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The Sky Lab 3 7

PARTIV:THE SKY DURING DIFFERENTSEASONS

Have you noticed that different stars are visible during different seasons? Let’s figure out why this is.

Look at the diagram below. Let’s ignore the rotation of the E ARTH around its own axis (only for now)and consider the motion of the E ARTH around the Sun.

Imagine that there are some aliens out there ― on a planet that is still unknown, but that is orbitingaround Arcturus. Arcturus is in the constellation Boötes and is visible in the summer. In fact, in theevenings in June it is the brightest star within about 20 degrees of your Z ENITH . So if you wanted, youcould wave to the aliens.

Half a year later, sometime in December, you want to wave at the aliens again ― but Arcturus is not up.Look at the diagram below, and convince yourself that the sun would blind you when trying to locateArcturus.

You can do this game with any other star too, and the sky appears to change with the seasons. Clearly,the stars do not move ― but the sky appears to move because of the E ARTH ’s motion around the sun.


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8 Lab 3 The Sky

SEASONS& CONSTELLATIONS

Let’s only consider stars that are in constellations along the ecliptic. The diagram below is the same asthe previous diagram. In June, at midnight, when you look towards the ecliptic, you see that Sagittariusis transiting, but if you want to look at Sagittarius in December, you would have to look through the sun,and you’d be blinded.


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The Sky Lab 3 9

WHICHCONSTELLATION IS UP?

1. The table below summarizes which constellation in the zodiac you would see at midnight.Complete the Table.

Month / Dateconstellation

transiting at midnightRIGHT ASCENSION of

that constellationSun would be seenin this constellation

June 21 Sagittarius 18 hours GeminiJuly Capricorn 20 hours Cancer

August Aquarius 22 hours LeoSeptember 21 Pisces 0 hours Virgo

October Aries 2 hours … November Taurus 4 hours …

December 22 Gemini … …January

FebruaryMarchAprilMay

CancerLeo………

……………

……………

2. Answer the following questions:

In March, which constellation would you be able to see best at midnight? _________________

What is the RA of that constellation? _________________

In May, which constellation would transit at midnight? _________________


Today, which constellation is transiting at midnight? _________________


In which constellation would you find the sun today? _________________


3. Do you get the idea? Explain below how RA correlates with the season.

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________ _________________________________________________________________________________

4. How many hours in RIGHT ASCENSION does each month correspond to? _________________

And one week corresponds to hour many minutes in RIGHT ASCENSION ? _________________


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10 Lab 3 The Sky

PARTVUSINGSKY CHARTSTO OBSERVE THESKY

You will be using all the information you learned in the last section to figure out in during which seasonand during what time of night you will be able to observe which stars. Use the constellation charts SC-1and SC-2. In particular check out SC-1. The x-axis corresponds to right ascension (RA), and the y-axisto declination (DEC). We said previously, that stars with a RA of 0 hours will be transiting on

September 21 at midnight. But let’s assume we want to observe at 8 p.m. at night, i.e., 4 hours beforemidnight. Thus stars with a RA of -4 hours, or with an RA of 20 hours (i.e., 24-4 = 20 hours) will betransiting.

Now check the SC-1 chart. On the x-axis you see “RA”. Find all stars that have an RA of 20 hours.Below “20h” on the x-axis you find a date (in a smaller script) - it says September 20. So at 8 p.m. in theevening of September 20, stars with an RA of 20 hours are transiting.

Using the previous Table and/or the SC-1 chart answer the following questions:[Note the SC-1 chart shows transits at 8pm, not at midnight!]

Write down today’s date.

Which stars are transiting at 8 p.m. today?

Which stars are transiting at 8 p.m. on August 21?

Give the name of a star that will transit today at 10 p.m.:

Give the name of a star that will transit on August 21 at 10 p.m.:

Which constellation is transiting at midnight on June 21?

Which constellation is transiting at 2 AM on June 21?

Which constellation is rising at 2 AM on June 21?

Which constellation is setting at 2 AM on June 21?

On which date will Capella transit at midnight?

On which date will it transit at 8 p.m.?

At which time will Capella transit today?

At what time, today, will Betelgeuse transit?

At what time, today, will the Andromeda Galaxy (M31) transit?


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The Sky Lab 3 11

QUIZ QUESTION 1(This is the type of question that could come up in a test)

Hold the picture at armslength right in front of you.Imagine that this picture is

part of the landscape in frontof you. Imagine that the topof the picture somewhatcurves around you. Imagethat the Horizon in that

picture corresponds to yourown Horizon.

Image taken from the AngloAustralian Observatory athttp://www.aao.gov.au/images.html

1. Where (at what latitude) was this picture taken? Explain how you arrived at that answer.[You may consult your textbook. Check out the diagrams with the star trails.]

2. Which hemisphere are you looking at? Explain.

3. The shutter of the camera was left open for several hours – for how long? Explain.


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12 Lab 3 The Sky

QUIZ QUESTION 2

1) Label the following quantities: theN ORTH and SOUTH C ELESTIALP OLES ; your HORIZON ; your ZENITH ;your N , S , E , W ; the MERIDIAN ; andthe C ELESTIAL EQUATOR .

2) Draw the paths of the five stars in thediagram. Also draw arrows showingthe direction of motion.

3) Indicate which stars are alwaysvisible, which stars are sometimesvisible, and which are never visible.

Always _______________________

Sometimes ____________________

Never ________________________

4) Take a red pen and draw the altitudeof Polaris (an angle). Then draw thelatitude of the observer in the picture.

5) Take blue pen and draw angle Polaris-to-Zenith and the angle Celestial Equator-to-Horizon. Whatcan you say about those angles?

6) Next add the altitude of Polaris to the angle Polaris-to-Zenith. What do you get? ________________

Then add your Latitude to the angle Polaris-to-Zenith. What do you get? ______________________

7) Compare both answers of question 6. What can you say about those angles?

8) What would happen to those angles as you travel North (i.e., as you increase your latitude?)

9) Comment on how the altitude of Polaris changes in relation to your latitude as you travel North.


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c) Outline a method of how you could measure that angle.

d) Your friend is taken aback but is not sure whether to believe you. So, you have to convince yourfriend that you are indeed correct. E XPLAIN WHY there is a correlation between your LATITUDE andthe ALTITUDE of Polaris (a diagram may also help).

3. Now you travel to Mexico. Explain in your own words how the sky in Mexico City looks differentfrom the sky in New York. Comment on the location of Polaris and on the paths of the stars at night.

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Skys Laboratory