Astronomy 11 No, this course isn t all about Star Wars
Earth s Rotation How fast are people on the equator moving? s=d/t =circumference/24 hours =(40,000 km)/24 hours =1670 km/h That s Mach 1.4! What if the Earth stopped spinning?
Why do we get seasons? The Earth s axis is inclined at 23.5º The Earth acts like a top and keeps the poles pointing in the same direction
Why is it winter in December? The Northern hemisphere is pointing away from the sun: less direct sun, fewer daylight hours It s summer in Australia in Dec since the Southern hemisphere is facing the sun!
Midnight Sun? North of the Arctic circle, we get 24h of daylight on the Summer Solstice The Arctic circle is 23.5º from the North Pole, so 66.5º N lat
Winter Sun vs. Summer Sun
Constellations
Who made up the constellations? For thousands of years, people have used the stars for a calendar and for navigation
Constellations Early people used constellations to tell stories and track the planets Constellations along the Earth s orbit (ecliptic) are called the zodiac We now use these to pinpoint the location of all celestial objects
Celestial Sphere
Celestial Sphere Just like the Earth has lines of latitude and longitude, we can pinpoint objects on the celestial sphere
North Star
Why isn t there a South Star?
Precession The Earth s axis also precesses like a top s In 10,000 years, Vega will be the North Star
The Moon
Maria Were called seas because ancient astronomers thought they were water Come from lava flows from the once liquid lunar mantle
Why so many Craters? Very little erosion No atmospheric protection No water Which ones are younger?
Why only one face? Revolution matches rotation Never see the back side of the moon
Lunar Effects Phases Tides Eclipses
Phases New Moon Waxing Crescent First quarter Waxing Gibbous Full Moon Waning Gibbous Last Quarter Waning Crescent
Does the Earth also go through Phases?
At new moon we can get a Solar Eclipse At full moon we can get a lunar eclipse Eclipses
During an eclipse we can see the corona And Flares
Not everyone gets to see a Solar Eclipse
They also don t come every month, due to the lunar orbit s tilt
What kind of eclipse is this?
Earthrise!
Lunar Eclipse!
Tidal Forces The force of gravity decreases with distance The moon experiences a gravity gradient This has slowed its rotation to match its revolution (29.5 days in a moonth ) It is also receding at 4 cm per year Similarly the Earth s rotation is slowing
Ocean Tides Water bulges in line with the moon The Sun also affects tides Spring tides are extra strong Neap tides are weaker
Distances to stars
Parallax When sighted from a different position, an object seems to change position since it is viewed from a different angle. Astronomers use the same principle to measure the distance to planets and nearby stars.
Parallax
We will perform a variation of this triangulation. If we walk along a perpendicular baseline then measure the angle, the distance can then be calculated from the tangent function: O Distance tan A Baseline so then distance can be found as Distance Baseline tan
Magnitude We define stellar magnitude as brightness: The brightest stars in the sky are classified as magnitude 1, then 2, to 5 for the dimmest stars we can see But are these brighter stars really brighter or just closer?
For this reason we separate: Apparent magnitude How bright a star looks in the sky Absolute magnitude How bright stars would look if they were all 10pc away
The distance ladder
How do we know the distances to stars and galaxies? For close stars, we can use parallax For distant stars and galaxies we need another method If only we knew the absolute magnitude of some reference stars...
Cepheid Variables These stars gases are unstable and so they expand and contract The period gives absolute brightness
Cepheid Variables The longer the period, the brighter the star This allows us to calculate the distance
Start R&D Questions Choose 7 of #1-10 p. 30 Also start Vocab definitions: choose 5 words from Summary p. 29-30