- If the Earth rotated in the opposite sense (clockwise rather than counterclockwise), how long would the solar day be?
- Suppose that the Earth’s pole was perpendicular to its orbit. How would the azimuth of sunrise vary throughout the year? How would the length of day and night vary throughout the year at the equator? at the North and South Poles? where you live?
- You are an astronaut on the moon. You look up, and see the Earth in its full phase and on the meridian. What lunar phase do people on Earth observe? What if you saw a first quarter Earth? new Earth? third quarter Earth? Draw a picture showing the geometry.
- If a planet always keeps the same side towards the Sun, how many sidereal days are in a year on that planet?
- If on a given day, the night is 24 hours long at the North Pole, how long is the night at the South Pole?
- On what day of the year are the nights longest at the equator?
- From the fact that the Moon takes 29.5 days to complete a full cycle of phases, show that it rises an average of 48 minutes later each night.
- What is the ratio of the flux hitting the Moon during the first quarter phase to the flux hitting the Moon near the full phase?
- Titan and the Moon have similar escape velocities. Why does Titan have an atmosphere, but the Moon does not?
Friday, October 30, 2009
Astronomers have confirmed that an exploding star spotted by Nasa's Swift satellite is the most distant cosmic object to be detected by telescopes.
In the journal Nature, two teams of astronomers report their observations of a gamma-ray burst from a star that died 13.1 billion light-years away.
The massive star died about 630 million years after the Big Bang.
UK astronomer Nial Tanvir described the observation as "a step back in cosmic time".
Professor Tanvir led an international team studying the afterglow of the explosion, using the United Kingdom Infrared Telescope (UKIRT) in Hawaii.
Swift detects around 100 gamma ray bursts every year
He told BBC News that his team was able to observe the afterglow for 10 days, while the gamma ray burst itself lasted around 12 seconds.
The event, dubbed GRB 090423, is an example of one of the most violent explosions in the Universe.
It is thought to have been associated with the cataclysmic death of a massive star - triggered by the centre of the star collapsing to form a "stellar-sized" black hole.
"Swift detects something like 100 gamma ray bursts per year," said Professor Tanvir. "And we follow up on lots of them in the hope that eventually we will get one like this one - something really very distant."
Another team, led by Italian astronomer Ruben Salvaterra studied the afterglow independently with the National Galileo Telescope in La Palma.
Little red dot
He told BBC News: "This kind of observation is quite difficult, so having two groups have the same result with two different instruments makes this much more robust."
"It is not surprising - we expected to see an event this distant eventually," said Professor Salvaterra.
"But to be there when it happens is quite amazing - definitely something to tell the grandchildren."
A GAMMA-RAY BURST RECIPE
Models assume GRBs arise when giant stars burn out and collapse
During collapse, super-fast jets of matter burst out from the stars
Collisions occur with gas already shed by the dying behemoths
The interaction generates the energetic signals detected by Swift
Remnants of the huge stars end their days as black holes
The astronomers were able to calculate the vast distance using a phenomenon known as "red shift".
Most of the light from the explosion was absorbed by intergalactic hydrogen gas. As that light travelled towards Earth, the expansion of the Universe "stretches" its wavelength, causing it to become redder.
"The greater that amount of movement [or stretching], the greater the distance." he said.
The image of this gamma ray burst was produced by combining several infrared images.
"So in this case, it's the redness of the dot that indicates that it is very distant," Professor Tanvir explained.
Before this record-breaking event, the furthest object observed from Earth was a gamma ray burst 12.9 billion light-years away.
"This is quite a big step back to the era when the first stars formed in the Universe," said Professor Tanvir.
"Not too long ago we had no idea where the first galaxies came from, so astronomers think this is a profound moment.
"This is... the last blank bit of the map of the Universe - the time between the Big Bang and the formation of these early galaxies."
Data from two powerful telescopes confirmed the result
And this is not the end of the story.
Bing Zhang, an astronomer from the University of Nevada, who was not involved in this study, wrote an article in Nature, explaining its significance.
The discovery, he said, opened up the exciting possibility of studying the "dark ages" of the Universe with gamma ray bursts.
And Professor Tanvir is already planning follow-up studies "looking for the galaxy this exploding star occurred in."
Next year, he and his team will be using the Hubble Space Telescope to try to locate that distant, very early galaxy.
Source: BBC News