Sept. 25, 2007 1

Apparent motions: Moon, Sun, stars & Orbits

• Review the daily (diurnal) vs. monthly (sidereal) sky

• How about the Moon and its phases?

• Celestial sphere needs a coordinate system: RA, DEC

• Re-visit the Sun, Moon, stars as projected on maps…

• Grand overview: orbits

Sept. 25, 2007 2

Enough Intro… How do the Sun & Stars “move”?

• Sun and stars rise in East, set in West, for their daily diurnal motion with period of Earth’s rotation, 24h

• Earth orbits the Sun with period 1y; a given star (e.g. Vega) thus rises ~4min earlier each night since the Earth has moved by ~1degree since 1deg =1/360 of full circle is ~1/365 of a year and since 1deg = 4min of diurnal motion (check: 360o = 24h; so 15o = 1h…)

a few figures (from your text, ch. 2) may help…

Sept. 25, 2007 3

Diurnal vs. Sidereal motion

Earth’s rotation (24h) causes diurnal motion of Sunand stars, which rise earlier in Boston than in LA…

Earth’s orbit around Sun (365d) causes sidereal motion of stars (and planets) which “move” at local midnight so that Summer Triangle is overhead at midnight in June vs. Orion in December, as seen from Boston

Sept. 25, 2007 4

So how do we “see” the night sky?

• Stars (e.g. Vega) change in elevation (angle above horizon) as they rise and set; but in fact are rotating about north celestial pole (near Polaris) as you will measure/verify in Evelab 1

Easiest to think of a Celestial Sphere of the fixed stars, with Earth rotating “underneath”. You will deduce this from Simple elevation measurements over 2 weeks of Vega, Big Dipper star, a Casseiopia star and Polaris in EL1.

Sept. 25, 2007 5

And why is summer hot? And reversed in N vs. S hemispheres?

• The Earth’s spin axis is tilted by 23.5o from plane of its orbit around Sun:

• Which also gives rise to

changing Sun max elevation

at noon measured in DL1

Sept. 25, 2007 6

How about the Moon?• All important daily vs. monthly effects: tides, moonlight• How do we understand what we think we know? Simple test of

your intuition: is Moonrise tonite later or earlier than last night? Why?

Sept. 25, 2007 7

Why “Man in the Moon”; Sidereal month on Moon?

• Moon’s “day” exactly equals Moon’s “year”: Moon rotates once per orbital revolution around Earth

• Moon’s synodic vs. sidereal month not the same:

• Similar effect for length

of solar “day” as seen

from Earth…(DL1 obs.)• Lunar eclipse not every

lunar month since Moon

orbit inclined 5o to ecliptic

Sept. 25, 2007 8

Celestial coordinate system: RA, DEC• Stars (and galaxies) “fixed” on celestial sphere, so give

them a longitude (RA) and latitude (DEC) coord.• We need to locate Sun, Moon, stars, galaxies on maps of

the sky (“finding charts” as in EL1…)

• RA = longitude coord., units of time (or degrees) with 0 in const. Aries

• DEC= latitude coord., with zero on Earth’s equator