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Astronomy

¨  How many eclipses happen in a year?
¨  Why does the north pole and north star always point north?
¨  What seasons can the constellation Ursa Major be viewed in from Denver?
¨  Does the earth rotate on its own axis or does it rotate up and down?
¨  Questions about ellipticity
¨  Can the Hellas Basin crater on Mars be viewed from the Earth?
¨  How can we imagine a universe without human beings?
¨  How would I draw a sketch of the relation between the Sun, Earth, and Moon?
¨  Streak of light across sky on September 22
¨  Why does Earth revolve around the sun?
¨  Why isn't Sirius considered part of the solar system?
¨  Has Phil Plait's Bad Astronomy website been taken over by hackers?

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QUESTION:
How many eclipses happen in a year?

ANSWER from Imagiverse on 21 April 2006:
Are you asking about lunar eclipses or solar eclipses?  Are you asking about total eclipses or partial eclipses or any eclipse of the sun or moon?  You can actually find this information by doing a little research on your own in books or on the Internet.  In google.com, you can type words like "eclipse frequency".  If you look up this topic in science books, you will discover what causes an eclipse.  Understanding the cause will help you figure out the answer.  If you still have a question, please rephrase your question by being more specific and submit it again.  Thank you for contacting us!

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QUESTION:
Why does the north pole and north star always point north?

ANSWER from Roger Herzler on 14 April 2006:
The "North Pole" is a human creation to describe one of the points on the Earth where the Earth's rotational spin on its axis is centered.  The other point is the "South Pole".  It is more a philosophical idea that it's "north" vs. "south" or any other name.  However, the Earth rotates on its axis and it has to be called by some name.

The "North Star" or "Polaris" is coincidentially located in the sky where the Earth's rotational center is projected into the sky.  It wasn't located by the North Pole a few thousand years ago.  In the future Polaris won't be the "North Star" in a few hundred years because it is moving through the galaxy just like we are.  It won't be located in the proper place to be called the "North Star" and won't be used as a good reference to the north pole as it is today.

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QUESTION:
What seasons can the constellation Ursa Major be viewed in from Denver?

ANSWER from Roger Herzler on 18 March 2006:
Ursa Major, part of which is known commonly as the "Big Dipper", is visible throughout the year in all seasons in the Colorado.  As the seasons go by it simply changes apparent position around Polaris (North Star) based on the same time every night.

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QUESTION:
Last night a thought came to me.  When the sun or moon sets it goes under.  That would mean that the earth does not rotate on its own axis but rotates up and down.  Think about it and if you know something about this, please respond.  I am 10 years old and I hope that what I think is true.

ANSWER from Bonnie J. Walters on 12 February 2006:
You have some very heavy thoughts!  You also stated your question clearly - good job.  Humans have wondered the same thing since ancient times.  I don’t know where you live on this wonderful planet called Earth, but I can assure you the Earth is spinning on its axis.  One of my favorite sites for young people your age is Harriet Witt’s Passenger Planet.  If you follow the link, it will take you to a superb demonstration of what you are doing while "riding" on the Earth. http://www.passengerplanet.com/yourearthride.html  I hope this helps you visualize what the Earth, Moon and Sun are doing.

Take care,
Bonnie J. Walters
Astronomer

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QUESTION:
I had some questions about ellipticity that I was hoping you might be able to help me with.  I noticed that the earth’s ellipticity was 0.00335.  How specific is this number?  For instance Mercury has an ellipticity of 0.000, is it really a perfect sphere?  And how can I get a more accurate number for all the planets?  Any help or direction you can provide is greatly appreciated.

ANSWER from Stephanie Wong on 5 February 2006:
According to the Fact Sheets in http://nssdc.gsfc.nasa.gov/planetary/, where it appears that you have gotten your info, they have listed the ellipticity as such.  Assuming that the website posts its info with consideration of the uncertainties in measurement, the value should be accurate to the final significant digit.  Nothing is a perfect sphere, but it appears Mercury is being close to being so.  Note that Saturn, which is quite oblate, has an ellipticity equal to 0.09796.  So, it wouldn't be surprising if Mercury's ellipticity is measured to be zero within experimental error.

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QUESTION:
Can the Hellas Basin crater on Mars be viewed from the Earth?  What size telescope would you need?

ANSWER from Roger Herzler on 13 January 2006:
Several features on Mars are visible in a backyard telescope.  For example, the snow caps are visible in a 4" telescope.

I was able to find some good pictures of the Hellas Basin, which is an impact crater:

Univ. of Texas:
http://www.tsgc.utexas.edu/everything/mars/hellas_basin.html

Imagiverse's own column on it:
http://www.imagiverse.org/activities/robotics/iro/aaron.htm

NASA website including details on it:
http://rst.gsfc.nasa.gov/Sect19/Sect19_12.html

and this Wikipedia entry:
http://en.wikipedia.org/wiki/Hellas_Planitia

According to Wikipedia, it was observed in the 1800s from Earth, and it wasn't uncommon for astronomers to be using 6-10" telescopes during that period.  My suspicion is that would be sufficient, but I've not viewed it personally and can't find a direct answer to "What size telescope would you need?"

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QUESTION:
How can we imagine a universe without human beings?

ANSWER from Stephanie Wong on 3 January 2006:
This question is quite a feat to answer, since if there were no human beings, no humans would exist to imagine a universe without human beings! :-D

When you think about something as grand as the universe, though, we humans are miniscule things, residing on quite a small speck of rock in the sky.  Consider this image on Astronomy Picture of the Day: http://antwrp.gsfc.nasa.gov/apod/ap060101.html

and then look at this "Family Portrait":
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-solarsystem.html

I hope this is a start for you to ponder your personal answer to this question.

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QUESTION:
How would I draw a sketch of the relation between the Sun, Earth, and Moon and then explain how do they move and on what timescale?

ANSWER from Roger Herzler on 1 October 2005:
This is raw opinion.  If you're doing this for an assignment you should ask your teacher if this is acceptable.

I believe that the best way to illustrate this type of relationship is to draw up a view from the "top down".  That is, looking down from above you can see the circular orbital paths that both the Earth-Moon and the Sun-Earth make.  If you needed to show how they move you could show rough estimates based upon how long they take to orbit a body (Moon and Earth) or maybe how long it takes for the Sun to complete one turn on its axis (~25.4 days).

For example, if you drew out a circle representing the Earth's orbit around the Sun, and placed 4 "Earths" on the circle - one for each equinox, you'd be able to illustrate that the Earth travels around the Sun.  You could label each Earth for the season its experiencing such as Fall, Winter, Summer and Spring.  It would form roughly an X if you drew a line between each of those seasonal Earths, with the Sun in the center.

A similar orbit diagram could be sketched out to show the Moon's orbit around the Earth, which occurs once about every 28 days.

Here's a decent orbital diagram that should help you get started on what it looks like:

http://www.nineplanets.org/overview.html
OR
http://ssd.jpl.nasa.gov/orbits_inner.html
(ignore the yellow asteroid belt)

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QUESTION:
Last evening (Sept. 22) at sunset, I saw an incredible streak of light across the sky.  It looked like some sort of rocket launch.  Can you tell me what it was?

ANSWER from Roger Herzler on 23 September 2005:
Do you live in California or a neighboring state like Arizona?  If so, you did see a rocket last night putting a satellite into orbit.  It was launched from Vandenberg Air Force Base.  I saw it as well and it was stunningly beautiful.  You can find more news about it here:

News Story:
http://www.signonsandiego.com/news/state/20050922-1958-ca-rocketlaunch.html

Vandenberg Launch Schedule:
http://www.spacearchive.info/

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QUESTION:
Please explain how it has been proved that the Earth revolves around the Sun once a year instead of the constellations traveling around the stationary Earth.

ANSWER from Roger Herzler on 27 September 2005:
Great question!  Is the answer "because Kepler said it was so" good enough? :o)

Actually, the answer can be found on this link:
http://curious.astro.cornell.edu/question.php?number=190

That link states that it's been proven in three different ways: "aberration (slight change in stellar positions due to Earth's speed), parallax (slight change in stellar positions due to Earth's changing position) and Doppler effect (slight change in color of stars due to Earth's speed)."

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QUESTION:
Why isn't Sirius considered part of the solar system?

ANSWER from Stephanie Wong on 26 September 2005:
For something to be inside the solar system, it has to be within the sun's heliosphere (i.e. within the "heliopause").  The heliopause is the boundary between the solar system and what we call interstellar space.  At the heliopause, the the sun's solar wind can no more push back the intestellar medium of interstellar space.  Sirius, which is a star over 8 light years away from the sun, is considerably outside the sun's magnetic bubble.  Therefore, it is not a part of the solar system, even though it is the brightest star in the sky.

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QUESTION:
Is it true that Phil Plait's Bad Astronomy website has been taken over by that other guy who makes up ridiculous claims about space exploration (such as trying to convince people that faces on Mars are other than optical illusions)?

ANSWER from Michelle Mock on 9 September 2005:
BadAstronomy.com is not to be confused with other sites using similar spelling attempting to purposely mislead the reader.  There are certain individuals in cyberspace who purposely create 'educational' websites intended to provide blatantly false information to anyone gullible enough to believe it.  Be careful of what you read online.  Just because you see it in print does not make it true.

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Last Updated:
13 May 2006
 

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