# For Wednesday, Jan. 28 Reading: Section 2.3 Assignments: Math Review (due today) Mini-Project #1 (due Mon. Feb. 2) Homework #1 (due Mon. Feb 2)

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For Wednesday, Jan. 28 Reading: Section 2.3 Assignments: Math Review (due today) Mini-Project #1 (due Mon. Feb. 2) Homework #1 (due Mon. Feb 2)

Thought Question If you look at this star in the south part of the sky, what direction would have a larger declination? A.higher in the sky B.lower in the sky C.to the east D.to the west HORIZON SWE

Winter Constellations TO SIRIUS (BRIGHTEST STAR IN SKY) visible in evening in winter ORION: (home to Betelgeuse and the Great Nebula) RA: about 5 h DEC: about 0°

Being a Good Astronomer Be able to predict: How stars will appear to move for different people on Earth How high a star can get in the sky (knowing RA and DEC) What time of year it is best to observe a star (knowing RA and DEC) ATMOSPHERE

A long photo of the northern sky:

A Good Telescope a star’s RA and DEC don’t change during night to keep pointing at one RA and DEC, most telescopes rotate with the sky TO NORTH STAR

Telescopes in Space In space, a telescope can continuously point at a star without needing to rotate

The Sky from San Diego Most stars rise in east half of sky, and set in west half: NORTH CELESTIAL POLE LOOKING SOUTH N W S E HORIZON circumpolar: stars that don’t rise or set for a person LOOKING NORTH  stars circle in 23h56m

The North Star altitude of North Celestial Pole (North Star) = your latitude!

The Sky from San Diego if lower part of circle doesn’t hit the horizon, the star is circumpolar… NORTH CELESTIAL POLE LOOKING SOUTH N W S E HORIZON the further the star is from a celestial pole, the bigger the circle it makes around the sky: LOOKING NORTH  stars circle in 23h56m 33°

Thought Question A star rises in the NE in San Diego. How long will it be “up” (above the horizon)? A.24 hours B.Between 12 and 24 hours C.12 hours D.Between 0 and 12 hours HORIZON NORTH CELESTIAL POLE N E W S

Star Motions A star on celestial equator (DEC = 0º) will: rise due E set due W spend exactly 12 hr above horizon NORTH CELESTIAL POLE N W S E HORIZON Exactly ½ the star’s path is above the horizon

The Sky from San Diego meridian: arc from north through zenith to south  stars always get to be highest in sky there… S N E W N E W S HORIZON NORTH CELESTIAL POLE MERIDIAN CELESTIAL EQUATOR ZENITH

Imagine cutting celestial sphere from N to S: The North Star altitude of North Celestial Pole (North Star) = your latitude! MERIDIAN NORTH CELESTIAL POLE CELESTIAL EQUATOR

A star’s maximum altitude depends on its declination and your latitude: Star Altitude MERIDIAN

Thought Question: What is the maximum altitude that a star with declination +15º can reach as seen from San Diego? (San Diego is at a latitude of about 33º.) A.18º B.33º C.42º D.48º E.57º F.72º G.The star is not visible from San Diego

Thought Question: What is the maximum altitude that a star with declination -25º can reach as seen from San Diego? (San Diego is at a latitude of about 33º.) OR: 0 (The star is not visible from San Diego) Enter value in degrees using the arrow keys (  to change number;  to change between digits), then hit SEND

Thought Question: What is the maximum altitude that a star with declination +40º can reach as seen from San Diego? (San Diego is at a latitude of about 33º.) Enter value in degrees using the arrow keys (  to change number;  to change between digits), then hit SEND

Thought Question: What is the minimum altitude that a star with declination +40º can reach as seen from San Diego? (San Diego is at a latitude of about 33º.) A.17º above the horizon B.7º above the horizon C.7º below the horizon D.17º below the horizon

For Friday, Jan. 30 Reading: review Section 2.3 Assignments: Mini-Project #1 (due Mon. Feb. 2) Homework #1 (due Mon. Feb 2) Telescope observing session on campus early next week?

Summer Constellations CYGNUS: lies in Milky Way RA: about 20 h DEC: about +40° Vega

What about the Sun? Earth’s motion around Sun slowly changes which constellations are visible at night:

Thought Question: You go out tonight (Jan. 28) and see a bright star in the constellation Cancer to the south at midnight. One week later at midnight this same star … A.will be somewhat southwest. B.will again be due south. C.will be somewhat southeast. D.won’t be visible (below the horizon).

The Sun’s Path ecliptic: Sun’s apparent path around celestial sphere during a YEAR  Sun’s RA and DEC change  planets, Moon stay near ecliptic Sun’s DEC Summer solstice (June 21)+23.5º Fall, spring equinoxes (Mar. 21, Sept. 21) 0º0º Winter solstice (Dec. 21)-23.5º

Solar and Sidereal (Star) Days SUN solar day: time between when Sun appears in same place in sky (24 hrs on average) sidereal day: time between when a star appears in same place in sky (23 hr 56 min) SUN Which type of day corresponds to exactly one rotation of Earth?

Thought Question: If the Earth orbited the Sun but did not rotate, which of the following would be true? A.The Sun would not rise or set for someone on Earth. B.A solar day would equal a year. C.A sidereal day would equal a year. VIEW FROM ABOVE EARTH N. POLE

Solar Time is NOT Star Time 1 sidereal (star) day = 23 hr 56 min = 0.997 solar day (4 minutes shorter than 1 solar day) Star time at sunset gets later by:  about ½ hr per week  about 2 hr per month (Earth moves 1/12 th of way around Sun) The star clock runs fast…

Right Ascension and Star Time use hours, minutes, and seconds of time for RA 1 h = 60 m 1 m = 60 s 0 h ≤ RA < 24 h RA increases going east Local Sidereal Time (LST): current RA on your meridian (“star time”)

The Sky from San Diego Looking S, you see stars up for 12 hrs or less NORTH CELESTIAL POLE N W S E HORIZON Looking N, stars are up for 12 hrs or more LOOKING NORTH increasing RA LST+6h LST-6h RA=LST

The Sky from San Diego Stars are rising in east half of sky, and setting in west half: LOOKING SOUTHLOOKING NORTH LST LST-2 h LST+2 h LST+4 h LST-4 h LST LST-2 h LST+2 h LST+12 h LST-6 h LST+6 h NE NW SE SW …like a clock where the face moves instead of the hands

Star time = RA on your meridian right now STEPS: 1.Start on the day that star time and solar time match up at midnight (Sept. 21) 2.Determine star time at midnight on the day you want 3.Determine star time for the clock time you want 4.Stars having RA within about 6 h of that star time will be visible Telling Star Time

Date:RA of Sun:LST at midnight: Spring equinox (March 21) 0 h (BY DEFINITION) 12 h Summer solstice (June 21) 6h6h 18 h Fall equinox (Sept. 21) 12 h 0 h REMEMBER THIS ONE!! Winter solstice (Dec. 21) 18 h 6h6h What’s Up? 1,2. Determine star time at midnight on the day you want

What Part of the Sky is Up? What is local sidereal time (LST) at midnight? Example: January 28 LST = 0h at midnight on fall equinox (Sept. 21) LST at midnight advances 2 h every month (and about 0.5 h every week) Jan. 28 is 4 months and 1 week after Sept. 21 LST  8h 30m

Thought Question: What will the star time be at midnight on May 14? A.3h 30m B.4h C.4h 30m D.15h 30m E.16h F.16h 30m

What Part of the Sky is Up? What is local sidereal time (LST) at 9pm? Example: January 28 LST at midnight  8h 30m 9 pm is 3 h earlier, so LST  5h 30m

How much of the sky can you see? 0h (MERIDIAN) -2h 2h SESW S

How much of the sky can you see? About 6h on either side of meridian +4h +6h +2h SE E NE

Thought Question: If the local sidereal time is 17 h and you are looking for a star having an RA of 14 h, which of the following is true? A.The star will reach its highest point in 3 hours. B.The star reached its highest point 3 hours ago. C.It isn’t possible to tell using the information given. increasing RA LST+6h LST-6h RA=LST

For Monday, Feb. 2 Reading: Section 2.4 Assignments: Mini-Project #1 (due Mon. Feb. 2) Homework #1 (due Mon. Feb 2) Mini-Project #2 (due Wed. Feb. 11) QUIZ #1 SCHEDULED FOR FRIDAY, FEB. 6 PLANETARIUM SHOWS: Tues. Feb. 3: NOON, 1 PM Wed. Feb. 4: NOON, 1 PM, 3 PM Thur. Feb. 5: NOON Mon. Feb. 9: 3 PM Tues. Feb. 10: 1 PM, 3 PM Wed. Feb. 11: 1 PM

Seasons on Other Planets?

Earth’s Orbit and Tilt Earth’s axis is tilted compared to the axis of Earth’s orbit

Why Does the Axis Matter? Interactive Figure In June: More than 50% of Sun’s light falls on N hemisphere You spend more than half a day on sunlit half of Earth (Sun is N of celestial equator)

The Sun’s Path ecliptic: Sun’s apparent path around celestial sphere during a YEAR  crosses celestial equator in two places  ONLY goes through zodiac constellations (and Ophiuchus ) Sun’s DEC Summer solstice (June 21)+23.5º Fall, spring equinoxes (Mar. 21, Sept. 21) 0º0º Winter solstice (Dec. 21)-23.5º

Thought Question: What is the minimum altitude the Sun can reach at noon in San Diego? (San Diego’s latitude is 33º.) (Enter your answer in degrees, rounded to the nearest whole number.)

Sun in Winter When Sun is S of celestial equator: daytime is less than 12 hr long Sun is low in sky at noon (33.5° on Dec. 21) rises in SE, sets in SW NORTH STAR LOOKING SOUTH: N W S E HORIZON

Summer S SESW Winter S SE SW

Sun in Summer NORTH STAR LOOKING SOUTH: N W S E HORIZON When Sun is N of celestial equator: daytime is more than 12 hr long Sun is high in sky at noon (80.5° on June 21) rises in NE, sets in NW

The Sun’s Altitude Sunlight coming in at a shallower angle gets spread over a greater area on Earth’s surface, and warms the surface less NORTH STAR N W S E HORIZON ALTITUDE

The Sun’s Altitude Sunlight coming in at a shallower angle gets spread over a greater area on Earth’s surface… how much greater? ALTITUDE ANGLE 90° ANGLE increase in area:

Would the Sun appear to change in size during the year? What season is it in the southern hemisphere during summer here? HYPOTHESIS 1HYPOTHESIS 2 (NOT TILTED) (TILTED) What if Earth’s orbit was like this: (TOP VIEWS) AND and Earth’s axis was like this: (SIDE VIEWS) Thought Questions: A.YES YES B.YES NO C.NO YES D.NO NO A.SUMMER SUMMER B.SUMMER WINTER C.WINTER SUMMER D.WINTER WINTER

Sun Appearance take a picture of Sun from the same place every few days at noon any noticeable difference in size of Sun?

Antarctic Ice map

Thought Question: What would seasons be like on this planet as it orbits the Sun? A.There would be seasons, but the temperature changes would be more extreme than on Earth. B.The seasons would be just like those on Earth. C.There would be seasons, but the temperature changes would be less extreme than on Earth. D.There would not be any seasons on this planet.

Seasons on Uranus

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