WHY IS THE WHY IS THE SKY SKY

DARK AT DARK AT NIGHT?NIGHT?

Ajou Uni. Division of Natural sciencesAjou Uni. Division of Natural sciences physics; Lee, Dong Myungphysics; Lee, Dong Myung

• 밤 하늘이 왜 어두운가에 대한 질문을 구하는 과정을 통해서 고대 천문학에서 현대 천문학까지 변화과정을 대략적으로 살펴본다 .

• 자연과학도로써 어떤 자세가 필요한가에 대해서 생각하는 기회를 가져본다 .

이번 세미나의 목적은…이번 세미나의 목적은…

• 1999.10.3 경북 보현산

The winter triangle The winter triangle

EgyptEgypt

Ptolemaeos Ptolemaeos

• Almagest

• A heavy thing falls faster than a light thing.

• Without force, things wasn’t moved

Ptolemaeos-Geocentric Theory Ptolemaeos-Geocentric Theory

• apparent motion

Galilei, Galileo Galilei, Galileo

• sunspot

• Galilean satelites

• The ring of Saturn

• “Eppur si muove”

Heliocentric theory Heliocentric theory

The Newton’s universeThe Newton’s universe

•The universe is infinite and static.

•The universe is filled with many starts infinitely and arbitrary.

The Static universeThe Static universe

The Olbers’ Paradox(1823)The Olbers’ Paradox(1823)

• In an infinite universe the light from each start should stream toward the Earth. You can see that successive “shell” of space contains a numbers of stars.Those stars’ combined brightness with many shells would then light up the night sky as if it were daytime.

* * 무슨 말이냐고무슨 말이냐고 ..??..??

** 아하 그렇쿠나…아하 그렇쿠나…• The surface of

sphere is proportion to the square of radius.

그거구나그거구나 .~~.~~

• The Light from each shell is L*S=constant ..

• There are many shells, because the universe is infinite.

• So,

<Some Answers>• Thomas Digges(1576):Distant stars were t

oo faint to see• Johannes Kepler(1600): A dark wall surrou

nds the starry space • Edmond Halley(1721): The light are not suf

ficient to move our sense• Jean-Philippe Loys de chesaux:Space is no

t transparent(1744)

The first correct resolutionThe first correct resolution

• Edgar Allan Poe• Light from distant starts fails to brighten the nigh

t because it has not had time to reach us;we can not see father than the universe is old

• The velocity of light is finite• In the Newton’s univers, Edgar Allan Poe’s solution fails; if the universe w

ere infinite, astronomers should see infinitely far.

• To modern astronomy……• Bessel, f.(1838) measures the distance of S

tuve by trigonometric parallax

Periodic variabilityPeriodic variability

period-luminosity relation

Absoute magnitude Absoute magnitude and distanceand distance

• 절대등급은 10pc 에 거리에 있을 때 가정하여 계산한 밝기 .

• 밝기는 거리 제곱에 반비례 한다 . •

• 우리 은하 밖에 외부은하가 있다 . -> 충격적 사실 .

Doppler effectDoppler effect

• light is the same• redshift->

• 관측자와 멀어질때 . • blueshift->• 관측자와 가까워질때

•

hubble –red shift(1929)hubble –red shift(1929)

허블 법칙허블 법칙• V=H*r

• The universe expands now.!!!

• -> 굉장한 충격 ..~!!

프리드만의 우주론프리드만의 우주론 (1922)(1922)

• 우주가 유한하다면 , 중력에 대한 평형을 이루러 면 , 우주가 팽창해야만 한다 .

• 아인슈타인도 처음에는 사실을 받아들이지않았다 .

• 팽창한다면 시간을 거슬러 가면 어떻게 될까… 모든 물질 사이에 작용하는 중력을 이길려면 초기에 무언가 필요하다 ..

• ->modern astronomy starts

Big bang!!

Why really is the sky dark at Why really is the sky dark at night???night???

1. the universe expands because it has not had time to reach us

2. The universe has too little energy

1996.1 Hubble Telescope1996.1 Hubble Telescope

I have questions~!!!