199922-Observing-Light-Pollution-docx

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Dec 6, 2023

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O b ser v i ng L i ght Pollu t ion B ig Idea : T he w a y w e vi e w the n i ght s ky v arie s greatl y due to both manmade lighting and the natu r a l v a ri at i on i n the br i ghtne ss of the Moon a s it goe s through pha s e s . Goal : Y ou will mea s ure the amount of light pollution from a lo c ation of y our c hoi c e. Then, you will de m on s trate ho w the tota l number of s tar s v i s ible to the e y e i s e s timated from s ample s . The s e m ea s urement s mu s t be repeat e d at lea s t three time s to in v e s tigate how the night sky b ri ghtne ss a nd the number of visi b l e s tar s c hange when the Moon i s v i s ible. R equirem en ts: Th r ee n i ght s , at l ea s t one wi th and one without bright Moonlight (Full moon o cc ur s on J un e 3, 2023). Tube of k no w n or mea s ured le ngth (e.g. paper towel tube mea s ured with a ruler - print a ruler if needed ). Pl ea s e r ead the ent i re pro j e c t before y ou go out to ma k e ob s er v ation s . It require s s ome p r epa r at i on, but noth i ng d i ff ic u l t. P hase I. G l ob e at N i gh t Fo r th is p r o j e c t, y ou will need to ma ke ob s er v ation s of the night sky on three different c lear n i ght s , at l ea s t on c e dur i ng a n i ght wh en the Moon i s v i s ible and on c e when it i s not v i s ible. C hoo s e a l o c at i on that is s afe and ea s il y a cc e ss ible and a v oid being dire c tl y near bright lig h t s s u c h a s s t re et l amp s . I MP O R T ANT : It ta k e s at l ea s t 10 m i n u te s for y our e y e s to full y adju s t to dar k ne ss (dilate). Loo ki ng at a sc reen or other li ght s ou rc e will c au s e y our e y e s to re s et ( c on s tri c t) meaning your e y e s will not be a s s en si t iv e and y ou will not ma k e reliable mea s urement s . 1. D e scri b e the l o c at i on w here y ou o bs er v ed the c on s tellation s . U s ing y our phone GPS o r Goog l e Map s , f i nd the l ong i tude a n d latitude and report them below. Ma k e a sk et c h of t he l o c at i on and i n cl ude i t be l o w . D e scri pt i on: The location is associated with moderate lighting free from busy roads with impact of traffic lights. Long i tude, Lat i tude: 32.767796, -96.796739 Sk et c h:
G l obe at Ni ght is an i nternat i ona l pro j e c t to mea s ure the impa c t of light pollution b y ha v ing “ci t iz en sci ent is t s mea s ure the i r l o c a l night sky brightne ss . 2. R ead about the effe c t s of li ght po l lution at https://www.globeatnight.org/light-pollution.php . W h ic h, i f an y , of the three ma i n t y pe s of light pollution are pre s ent at the lo c ation where you a r e m ak i ng ob s er v at i on s ? W hen w e ob s er v e s tar s ( wi th our e y es alone or with opti c al aid s u c h a s tele sc ope s ), we ob serve the ir b ri ghtne ss e s . As tronomer s ha v e a s omewhat c ompli c ated sys tem c alled s tellar “m agn i tudes” to de sc r i be th is br i ghtne ss . The magnitude sys tem wa s de v eloped b y the Gr eek Hi ppa rc hu s , w ho liv ed around 150 B .C. He di v ided s tar s into c ategorie s on the ba s i s of the ir b ri ghtne ss , a ssi gn i ng "f i r s t ran k " to th e brighte s t s tar s and "s e c ond ran k , " " third ran k , " and so on to s u cc e ssiv e ly fa i nter one s . A s tar' s r a n k ing ha s c ome to be k nown a s it s apparent magnit ude . Late r , a s t r onomer s made th is sys tem m ore quantitati v e, defining that a differen c e of fi v e m agn i tude s repre s ent s e x a c t ly 100 t im e s the differen c e between two s tar s ' brightne ss e s . T here a r e about f iv e magn i tude s bet w een th e brighte s t and fainte s t s tar s that we c an s ee, and th e sc a l e is l ogar i thm ic wi th ea c h s tep” in magnitude repre s enting a c hange in brightne ss b y a fa c to r of ab o ut 2.5. T he magn i tude sys tem i s al s o odd in that it i s deri v ed from a ran k ing (fir s t is b ri ghte s t, et c .) s o the br i ghte s t s tar s ha v e a low magnitude number ( s ome e v en being negat ive) and tho s e wi th h i gh magn i tude are v er y dim. A u s eful graphi c made b y the Uni v . of Virginia, de m on s t r ate s the magn i tude sys tem b elow. A u s efu l w a y to c hara c ter iz e the dar kn e ss of y our night sky i s to identif y the fainte s t s tar s t ha t y ou c an c on sis tent ly s ee. T h is is c a ll e d the “limiting magnitude.” Remember, y ou need to b e out si de fo r at l ea s t 10 m i nute s to i dentif y the fainte s t s tar s v i s ible in gi v en c on s tellation s . M agn i tude c hart s for i nd ivi dua l C on s tellation s are a v ailable on the Globe at Night website . C hoo s e a c on s te ll at i on that will be vis ible when y ou plan to ob s er v e. (If y ou are not s ure w hich c on s te ll at i o ns will be up, u s e Stellarium to c he ck .) Sele c t the c lo s e s t Latitude for y our lo c at ion (i f y ou a r e i n Fl or i da, that will be 30- N ). Print out the magnitude c hart s or download them to a de vic e wi th “n i ght visi on mode” (e.g. enable red tint in iOS ). If y ou don’t ha v e a cc e ss to a p rin t er , y ou c an s tart a sk et c h of the br i ght s t a r s s hown and “fill in” y our sk et c h with fainter s tar s w hen y ou The common type of lighting pollution common in the chosen location is light glare. The light glare is associated with vehicles vising the location at night.
a r e obser vi ng. If y ou ha v e troub l e s pott i ng the c on s tellation s , u s e the helpful guides on Globe at Night or do w n l oad a free augmented rea li t y s tar c hart app for y our phone (e.g S kyv iew Lite). If the M oon is visi b l e, k no wi ng w h ic h c on s te ll at i on s are near the Moon ma y help y ou orient y our s elf. 3. R e c o r d y our ob s er v at i on s be l o w for at lea s t three night s , and submit them to Globe at Night . Als o t ry t o take a pho t o of the c on s tellation s with y our phone or c amera and in c lude it be l o w . Observing Log f rom y our fi r st n i ght Date 21 st June Time Longitude 32.767796 Latitude -96.796739 Sky Conditions Partly cloudy Ho w d ark was your sky that night? C ons t ella t i on Limiting Magnitude 1 6.5 2 7.5 3 15.8
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Observing Log o f y our sec ond n i gh t Date 22 nd June Time Longitude 32.767796 Latitude -96.796739 Sky Conditions Partly cloudy Ho w d ark was your sky that night? C ons t ella t i on Limiting Magnitude 1 6.5 2 4.5 3 11.8 Observing Log o f y our t h i rd n i gh t Date 23 rd June Time Longitude 32.767796 Latitude -96.796739 Sky Conditions Partly cloudy
Ho w d ark was your sky that night? C ons t ella t i on Limiting Magnitude 1 6.5 2 9.5 3 13.8 4. Av e r age the n i ne v a l ue s y ou obta i ned for limiting magnitude in three dire c tion s on the th ree n i ght s . T h is is y our mea s ured v a l ue quantif y ing y our lo c al night sky brightne ss . Citie s have limi t i ng m agn i tude s of 3 or 4. A “d a r k sky c on s idered e xc ellent for s targa z ing will ha v e a limi t i ng m agn i tude of 7 or better. The average is 9.16.
P hase II. E sti m ati ng t h e numb e r o f v isible stars in the sky Fo r a qu ick but rea lis t ic i dea of ho w man y s tar s a per s on c ould s ee without a tele sc ope, a s t r ono m e r s u s e a s tat is t ic a l method c alled s ampling, in whi c h the number of s tar s in s om e k no w n f r a c t i on of the sky ' s area are c ounted; then that number i s "sc aled up " to gi v e an e s t im ate of the number that w ou l d be vi s ible a c ro ss the entire sky . Almo s t a s important a s this tota l nu m ber is i t s un c erta i nt y , w h ic h can be c al c ulated from how c lo s el y the re s ult s from d i ffe r ent s amp l ed l o c at i on s agree wi th ea c h other. I m ag i ne l oo ki ng out at the sky through a hollow tube. The ra ys of light entering the tube c o me f r o m a m u ch l arger ci r c u l ar s pot on the sky . The tube, if mo v ed in all dire c tion s , would e v ent ually c o v e r the s urfa c e of an ( i mag i nar y ) hemi s phere c entered on y our head; tho s e dire c tion s , e x tended, w ou l d form the "dome of the night sky '' abo v e y ou or an y c lear night. 5. C a r efu lly mea s ure the l ength of the tube, L , and it s inner diameter, D . Re c ord the s e bel ow . 6. W hat is the pre cisi on to w h ic h y ou ha v e mea s ured L and D ? Ma k e s ure that the numbe r o f si gn i f ic ant d i g i t s y ou u s e to e x pre ss y our mea s urement s ma k e s s en s e. The length of tube L has been measured considering the eye of the observer. The a r ea of the open end of the tube is ju s t the area of a c ir c le of diameter D , whi c h i s : 𝐴 ? ? ? = π( 𝐷 /2) 2 = π 𝐷 2 /4 The length of tube L is approximately 43 cm. the diameter of D is approximately 4 cm.
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= = = = 2 If y ou w e r e to mo v e the tube a ll aroun d abo v e y our head, the far end would tra c e out the im ag i na ry hem is phere s ho w n i n the f i gure abo v e. The radiu s of thi s hemi s phere would be t he l ength L of t he tube. T he s urfa c e area of the imaginar y hemi s phere i s half the s urfa c e area o f a c o m p l ete s phere of rad i u s L : 𝐴 ? ?? ? ? ℎ?? ??? ℎ? ? ? = 2 π 𝐿 2 The a r ea at the end of the tube, e x tended and enlarged, proje c t s out onto the sky , a s s ho wn . The im ag i nar y hem is phere, e x tended and enlarged, al s o proje c t s out a s the hemi s phere of sky s een o v e r an ob s er v er. S o w e c an s a y that a proportion or ratio e x i s t: 𝐴??? ?? ℎ?????ℎ??? ?? ??𝑦 𝐴??? ?? ????????? ???? 𝐴??? ?? ????????𝑦 ℎ?????ℎ??? 𝐴??? ?? ??? ?? ???? ? ? (2π𝐿 2 ) 8 𝐿 2 (π𝐷 ) 2 /4 𝐷 2 7. C a lc u l a t e the rat i o of the area of t h e imaginar y hemi s phere o v er the area of the end of the tube and re c ord i t be l o w . The Area of the hemisphere is 100.544 La s t ly , w e ma k e an a ss umpt i on: that s tar s are s pread e v enl y a c ro ss the night sky . If that i s t rue , then the m ore area w e s amp l e, the m o re s tar s we s hould s ee. Mathemati c all y we ha v e the ra t io : 𝑇???? ?????? ?? ????? ?? ℎ?????ℎ??? ?? ??𝑦 𝑁????? ?? ????? ?? ????????? ???? 𝐴??? ?? ????????𝑦 ℎ?????ℎ??? 8 𝐿 2 𝐴??? ?? ??? ?? ???? 𝐷 2 8. Usi ng t h e rat i o s w e ’v e j u s t de sc r i bed, write a formula for the total number of s tar s in th e sky that depend s on the number of s tar s within the proje c ted s pot (on a v erage) and the ratio 8 𝐿 2 . 𝐷 = 8L 2 /D 2 = 80,000 stars 9. N o w y ou are read y to ma k e y our o b s er v ation s and appl y the abo v e formula to e s timat e t he nu m be r of s tar s i n the sky . R emember to allow y our e y e s at lea s t 10 minute s to adju s t t o t he l o w li gh t . Loo ki ng through y our tube, c ount how ma y s tar s y ou c an s ee. Repeat thi s for 10 pa r t s of the sky , be i ng s ure to s ample in all dire c tion s . Re c ord both the dire c tion of the sky and the number of s tar s that y ou c ounted in the table below. Al s o, note the time and d a t e o f y ou r ob s er v at i on s and w hether or not the moon i s v i s ible. Time / D a te : Is t he M oon visi b le ? : ___ y e s / no ___ _
Region of Sky 1 2 3 4 5 6 7 8 9 10 Average Direction East East West North South South North West East West # of Stars 10 13 8 10 11 12 14 10 13 9 11 T i p s fo r y our s tar c ount i ng:
2 2 Loo k a li tt l e out of the si de of y our e y e when c ounting s tar s . Thi s te c hnique, c alled a v e r ted visi on, l et s the e y e ha v e a little more s en s iti v it y to faint s tar s than loo k ing di rec t ly at t h em w ou l d (and norma lly , when y ou ga z e at the s tar s , y our v i s ion i s n't re s tri c ted by a tube, s o s ome s tar li ght w ou l d e nter the s ide s of y our e y e s ). T ry to c o v er a l ot of d i fferent d i e c tion s and altitude s , from loo k ing s traight up ( z enit h d ir e c t i on) to l oo ki ng near the hori z on and toward ele v ation s in-between ; thi s will gi ve y ou a more rea lis t ic e s t i mate of the s tar number s . Y ou no w ha v e mea s ured the a v erage number of s tar s y ou c an e x pe c t to s ee in y our tube, b u t ho w a cc u ra te is th is v a l ue? In sci en c e quantif y ing the po ss ible error in y our mea s urement i s jus t a s im po r tant a s the mea s urement i t s elf. B y c han c e y ou ma y get e x a c tl y the c orre c t an s we r , bu t i f y ou r e x per i ment is not v er y a cc urate, y ou c annot e x pe c t an y one el s e (in c luding y our s elf, i f you t ri ed aga i n ) to f i nd the s ame re s u l t. Co n c lu s ion s are drawn from e v iden c e that i s repeatable. 10. C a lc u l a t e the sc atter, or un c erta i nt y , in y our a v erage number; the be s t c hoi c e i s the s tan dard de vi at i on, σ . T he s tandard de vi at i on of the s ample y ou ha v e ta k en c an be c omputed us ing Exc e l (“ = STDEV . S ()” ), Goog l e S h e et s (“ =STDEV()” ), or an online calculator . The a v erag e nu m be r of s tar s s een through the tube, then, c an be e x pre ss ed a s a number with an un c e r ta i nt y : a v erage ± σ . R e c ord thi s below. The standard deviation = 2.309, the range is 11 ± 3 = (8 < x < 14) 11. W hen the a v erage number of s tars i s multiplied b y the ratio 8 𝐿 2 y ou will obtain an e s timat e o f 𝐷 the tota l number of s tar s that that s hould be v i s ible to the na k ed e y e at an y one time. T he un c e r ta i nt y i n that tota l nurnber is j u s t σ multiplied b y the s ame fa c tor, 8 𝐿 2 . Re c ord the t o t al 𝐷 nu m be r of s tar s and un c erta i nt y fr o m y our mea s urement s below. = 80,000 12. H o w do y ou e x pe c t the pre s en c e o f the Moon to c hange the number of s tar s that are v i sible to y ou r ey e at n i ght? S tate a te s table predi c tion that y ou c an te s t u s ing thi s pro c edure. 13. N o w r epeat the e x per i ment on at l ea s t two more da ys when the c ondition s ha v e c hang ed so that the Moon is no w visi b l e/not vis ible. Re c ord y our ob s er v ation s and c al c ulate the tot al nu m be r of s tar s visi b l e i n the sky . Time / D a te : The presence of the moon will cause light pollution, therefore reduce the number of start that are observed.
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Is t he M oon visi b le ? : ____ y e s / no __ _ Region of Sky 1 2 3 4 5 6 7 8 9 10 Average Direction East East West North South South North West East West # of Stars 6 7 8 5 6 7 9 5 5 7 7 N u m be r of s tar s ± un c erta i nt y = 4 or 10 Time / D a te : Is t he M oon visi b le ? : ____ y e s / no ____ Region of Sky 1 2 3 4 5 6 7 8 9 10 Average Direction East East West North South South North West East West # of Stars 13 13 13 13 11 13 13 12 9 10 12 N u m be r of s tar s ± un c erta i nt y = 9 or 15 14. B a s ed on the data y ou ha v e c o ll e c ted, are y ou able to c onfirm or reje c t y our predi c tion f or ho w th e Moon c an affe c t the number of v i s ible s tar s ? Might there be other fa c tor s to e xplore that c ou l d c ontr i bute to d i fferen c es in the number of s tar s y ou c ount? E x plain y our rea soning and p r o vi de s pe ci f ic e vi den c e fro m y our ob s er v ation s to s upport y our rea s oning. The moon reflects a more light therefore distract the observation. The number of starts recorded when the moon is present is slightly lower than the number of starts when there is no moon.