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Astronomy

¨  What would happen if the sun disappeared for an hour?
¨
  What is the correct date for the last Mars closest approach to Earth?
¨
  Who/what was the Earth named after?  What is its astronomical symbol?
¨
  Does the Earth spin fast or slow?
¨
  What would happen if all the planets were the same size?
¨
  What is a clear sky clock?
¨
  What does the Big Dipper look like in all four seasons?
¨
  Are other stars as large as the sun even though they look so small in telescopes?
¨
  What are shooting stars, and why do we "wish upon them"?
¨  Is it true that stars twinkle and planets don't?
¨
  Is there such a thing as a leap second?  Is it significant to the astronomer?

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QUESTION:
What would happen if the Sun disappeared for an hour?

ANSWER from Phil Plait on 16 November 2003:
If the Sun disappeared for an hour, the temperature on the Earth would drop, but not much; maybe a few degrees.  But the oceans don't change temperature very quickly, so it would take a while for the Earth to freeze over.  If the Sun came back in an hour, things would quickly return to normal.

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QUESTION:
For the Mars closest approach on August 27, 2003, it has been stated in most publications that it's been about 60,000 years since Mars has come so close to Earth.  However, some predictions have it at around 100,000 years.  I have heard that depending on the models used to predict orbits that the exact date of last closest approach varies.  Can you explain?  Is there a "most probable" date?

ANSWER from Jane Luu on 15 August 2003:
I consulted with my colleague Gareth Williams at the Smithsonian Astrophysical Observatory, who is an expert on solar system orbits, and he said the following.

He had not heard of the 100000 years date, but it may be consistent with the 60000 years date.  It is entirely possible that there may have been a smallest closest approach 100000 years ago, with the approach 60000 years ago being somewhat further away, but still close.

 As for various models, our imprecise knowledge of the position of Jupiter -- which strongly affects Mars's orbit -- is one source of uncertainty in the positions of the planets in the past (or future).  Another complication for Mars is that its motion is affected by the asteroid belt nearby.

Jane Luu
Astronomer
MIT

ANSWER from Mark Marley on 15 August 2003:
Kepler solved the "2 body" problem 400 years ago.  But there is no analytical solution for the orbits of the 9 planets when you have to keep into account all the mutual pulls and tugs (Jupiter on Mars, Venus on Earth, etc.).  So computer programs are used that add up all the forces and adjust the mathematical trajectory over time.  This is done thousands of times for each orbit of Earth around the sun and then you just step forward or backward in time to figure out what happened in the past or will happen in the future.

The fact that different codes give different answers demonstrates that Mars is indeed very close this year, but its only closer by a relatively small fractional amount than it has been at other "close" approaches over history.

I don't know the codes in question, but I would bet there is not simple answer to the question, which just goes to show we don't know as much as you would think!

Mark Marley
Astronomer
NASA Ames Research Center

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QUESTION:
Who or what was Earth named after?  What is the symbol for Earth?

ANSWER from Stephanie Wong on 8 May 2003:
Unlike the names of the other planets, which in English are named after the characters in Roman mythology, the word "Earth" comes from the term referring to the ground, soil, or as what is "below" the heavens, and a whole assortment of other related meanings.  The early word, in one form of spelling, erthe, is from Old English, in the 11th century.  Use of the word to mean the planet that we are upon originated in the 15th century.  So, the Earth was not named after anyone/thing, but was a word that enlargened from simply meaning the ground to the designation of this planet.  We have kept the multitude of archaic definitions and that is why Earth is such an expansive term today.

(Sources: Merriam Webster Dictionary, Oxford English Dictionary)

The symbol commonly used to denote Earth is the circle with two perpendicular lines (a circular hot cross bun, if I may analogize) through the diameter of it.  If you can read the Symbol font, it is this character: [ Å ].  This symbol is sometimes used in the sciences to associate a variable/constant with the Earth.  For example, M Å (M with the subscript symbol) is often used to denote the mass of the Earth.

Stephanie Wong
Alberta, Canada

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QUESTION:
Does the Earth spin fast or slow?  How and why?

ANSWER from Stephanie Wong on 21 March 2003:
That depends on what you mean by fast and slow.  To make a crude approximation, take the Earth, rotating once in 23.9 hours.  The radius of the Earth at the equator is 6,378 kilometres.  So,

Circumference of Earth = 2(pi)(radius) = 40,074 kilometres

Speed = circumference/time = 1678 kilometres/hour

That's pretty fast! This is the speed of a point on the equator as it rotates about its axis.  As you go nearer and nearer to the poles (or go inside the Earth), the speed will decrease, since you are rotating about a smaller radius (think of dividing the Earth into many thin discs.  Sweeping out a smaller circumference in the same amount of time results in less speed.  So, depending on what you think is fast, and the reference frame from which you measure Earth's speed, you can decide whether the Earth moves fast.  For example, standing on Earth now, I don't sense the Earth's movement because I am moving equally as fast with the Earth.  So is the Earth fast or slow?  Compare to a car moving at around 60 km/hr or the speed of light at 1.08 billion km/hr!

The Earth spins because Earth formed from a spinning cloud of gas and dust.  The early solar system was a large cloud of gas rotating about the proto-sun.  As the cloud condensed, gravitational concentrations coalesced into what we call planets.  These objects still contain the angular momentum they had at formation.  The rate of spin has changed over time, due to many factors, one of which includes the moon's formation and orbit around the Earth.  It is common to have spinning objects in the universe, rather than non-spinning objects.  I refer you to a website to delve into the spinning of planets in more detail:
http://lep694.gsfc.nasa.gov/lepedu/whyspin.html

How does it spin?  The Earth is approximately a "rigid" body suspended in a near-vacuum space.  Once set into motion (at time of formation), it merely rotates about its axis as it did billions of years ago, maintaining the same axis (imagine a spinning top).  It wobbles a bit (like a top), but maintaining its speed, it continues to rotate for an indefinite amount of time.

Stephanie Wong
Mathematics Student
Alberta, Canada

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QUESTION:
What would happen if all the planets were the same size?

ANSWER from Wendy Wooten on 6 February 2003:
The size of the planets would affect the gravitational field around them.  The force of gravity is directly proportional to the mass of the body that is setting up the field.  If the planets all became as big as Jupiter or Saturn, gravity on the other planets would increase.  It would not increase so much as to have an effect on the other planets, but the moons that orbit the planets would have a greater force on them, so they would have to travel faster to stay in orbit or else they would fall into the planets.  On the other hand, if the planets were all as small as Pluto, they would have less gravitational force on their moons, and they would have to orbit slower or else they would travel off into space.

Wendy Wooten
Educator
Southern California

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QUESTION:
What is a clear sky clock?  What parameters does such a "clock" measure?

ANSWER from Roger Herzler on 4 February 2003:
A "Clear Sky Clock" is an online tool provided through the skills of Attilla Danko and the data from Canadian Meteorological Centre.  At present this tool shows the user the approximate cloud cover, transparency, seeing and darkness for given hours of the day and locations, such as observatories or star party locations.

Let's define some astronomy terminology.  The term "transparency" is how transparent or clear the sky is from the ground to space.  It is driven by moisture and dust in the air.  The term "seeing" is the turbulence of air currents in the sky which also affects observing, especially at higher telescope magnification.

The Clear Sky Clock is designed to predict these observing conditions for astronomers, which are critical to any observing trip.  It becomes very difficult to turn telescopes to the skies and see celestial objects if there is cloud cover, or poor transparency and seeing.  Anyone around the world can request a clock for their own observing area if a Clear Sky Clock isn't set up for an area nearby.  This is done by getting the observing site's latitude and longitude, and then following the instructions on the website.

External links:

Clear Sky Clock Homepage
http://www.cleardarksky.com/csk/

Canadian Meteorological Centre
http://www.cmc.ec.gc.ca/cmc/htmls/mainpage.html

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QUESTION:
What does the Big Dipper look like in all four seasons?  Is there a good website displaying its positions?

ANSWER from Dave McCarter on 2 November 2003:
There are four seasons, so the sky is shifted ninety degrees each season.  Right now we are in the middle of fall, which I think means the fall of leaves, rain and snow, and I noticed that the handle of the Big Dipper is horizontal in the north-west sky by 19h (7pm).  By mid-winter, late January or early February, the Big Dipper will be standing on its handle in the north-eastern sky.  In mid-spring, late-April or early-May, the Big Dipper will be arching overhead with the handle almost west to east.  Then by mid-summer the Big Dipper will be handle up high in the western sky.

ANSWER from Roger Herzler on 22 January 2003:
Good question.  In fact, the appearance of the Big Dipper, which is an asterism, not a true constellation, does change from our perspective throughout the year.  This change is due to the orbit of the Earth around the Sun, and our tilted axis.

If the Earth didn't have a tilted axis we'd see the same stars all year long.  However, as we move around the Sun we are presented with different stars.  This change is what makes the Big Dipper look different to us.  The stars themselves don't change position from our perspective.  This is better explained visually, so I would suggest asking a science teacher, or you could see pictures of it on the web (refer to links below).

Unfortunately, I can't find a specific website that shows you pictures of the Big Dipper in each season.  I have an idea though: Why don't you sketch the Big Dipper and how it looks from your house, maybe every 4 months and compare them.  The time of night you do this should be somewhat the same for each drawing, say 8:30PM, and then you can see the change for yourself.  This idea would take a little planning on your part, but with a calendar, a pencil, paper and your eyeballs you could sketch it out and take part in a little astronomy at home.

External web links:

Discussion of Earth's seasons (and tilt)
http://www.usatoday.com/weather/tg/wseason/wseason.htm

Ursa Major (the actual constellation the Big Dipper is in)
http://www.astro.wisc.edu/~dolan/constellations/constellations/Ursa_Major.html

Roger Herzler
Amateur Astronomer
Southern California

ANSWER from Bonnie J. Walters on 26 January 2003:
This is a good question and there are several important concepts to consider.  Which direction does the Big Dipper's position change during the seasons and WHY does it change during the year?  I will give you the position of the Big Dipper at this moment.  Then, I will clue you in on how to figure out where it will be any time of the year.

First off, remember the Big Dipper's real name is Ursa Major.  Use the two "pointer stars" from the end of the bowl to find Polaris, the north star.  Polaris designates the celestial north pole, the point in the sky all the constellations revolve around.  Why do they revolve around this point?  Because the Earth is turning, or revolving on it's axis.  The Earth revolves from west to east; the stars will rise in the east and set in the west.  So throughout one night, Ursa Major (and all the other constellations) will rise up from the east and set in the west.  All the constellations turn "counter-clockwise" around Polaris.  However, during the year, if you check the night sky at the SAME time each night (say at 2100) the Ursa Major's position will change.  This is because the Earth is orbiting the sun.

Tonight, January 26, if you look to the celestial north pole at night you will see Ursa Major is to the right of Ursa Minor.  Sketch both Ursa Minor and Ursa Major on a piece of paper and be sure to include the horizon.  Then divide the sky into quarters.  The perpendicular and horizontal axis should cross through Polaris.  Your horizontal line should be going through Ursa Major tonight.  Each quarter stands for a season.  What's our current season?  Winter.  So can you figure out what position Ursa Major would be in spring?  In summer?  In fall?

There is a very easy tool for finding the positons of any constellation at any time of the year.  It's a planisphere!  Planispheres are very easy to use...  First you dial in the date (month and date) and the time you want to go observing.  Next you hold the planisphere above your head with the north horizon pointing north, the other compass directions will follow - right?!  Then, presto!  The constellations will be above you as they will appear at that time in the sky!  Who needs fancy computer programs!

Good luck and clear skies!

Bonnie J. Walters
Amateur Astronomer
Volunteer Telescope Technician
Telescopes in Education
Mt. Wilson, California

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QUESTION:
Are the stars as big as the sun even though they look smaller they are more far away?

ANSWER from Bonnie Walters on 21 November 2002:
A very good question!  I struggled forever with it!  I'm going to answer it the easy way otherwise the subject gets very complicated real quickly.  I would be willing to go more in depth but only if you write back with a really specific question with a specific topic.  Of course the research was a good exercise for me!  I now know more about luminosity, mass, temp, HR diagrams, parallax etc. than I ever have.

The simple answer to your question is YES!  Stars come in all sizes - from very small but super bright neutron stars to gigantic red-giants that are bright because they are so massive.  They look like little pin points of light because they are so far away.  Astronomers can measure many things about stars.  They can measure stellar distance, temperature, luminosity (brightness), mass, and spectra among other things.  Sometimes if the star is big enough, bright enough and close enough, their size can even be measured directly!

Bonnie J. Walters
Amateur Astronomer
Telescopes In Education
Mt. Wilson, CA

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QUESTION:
I was wondering if you could tell me what really are "shooting stars" and why do we "wish upon them"?

ANSWER from Bonnie Walters on 7 October 2002:
What a wonderful question! Shooting stars are not really stars at all but little pieces of space dust.  We call them meteors when they leave a streak of light through the sky.  The are actually burning up in the atmosphere.  People called them shooting stars because they looked like a star falling.  Just remember most of those bright streaks you see are from an object no bigger than a grain of sand.  As for why do people wish upon them?  I don't really know the answer.  I was always told to make a wish when I saw one.  Perhaps because they seem so magical... people started the tradition a long time ago.  Walt Disney especially contributed to the myth with "Wish Upon a Star."  No matter what anyone says, I still make that wish when I see them.

Bonnie J. Walters
Amateur Astronomer
Telescopes in Education
Mt. Wilson, CA

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QUESTION:
Is it true that stars twinkle and planets don't?  If it is because of the atmosphere, then how do you make out that it is the planet Venus and not a bright star?

ANSWER from Roger Herzler on 11 December 2002:
In reality the light from planets is also affected by Earth's atmosphere.  The turbulent atmosphere that you noticed is called "seeing" by astronomers.  "Poor seeing" is especially bad turbulence and heavy twinkling.

Seeing is especially noticeable when you view planets in a telescope.  For example, when Jupiter is viewed through a telescope you can see the waves of turbulence roiling around in the eyepiece.  The effects of turbulence become even worse as you increase the magnification of the telescope.  This is the main reason there is a maximum usable magnification for every Earth-based telescope.

The fact that planets don't tend to "twinkle" as much as stars is probably a result of their being brighter, making it more difficult to see the turbulence surrounding them.  Brighter stars also seem to be less affected by seeing than dim stars.  However, this is just perception.  All light is affected by our atmosphere.

Roger Herzler
Amateur Astronomer
San Diego, California

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QUESTION:
Is there such a thing as a leap second?  If so, does it have any significance to the amateur astronomer?

ANSWER from Bonnie Walters on 30 July 2002:
Believe it or not there is such a thing as a leap second!  It is periodically (from one to two years) inserted in the atomic Coordinated Universal (UTC) time scale.  It serves to keep the difference between UTC and Earth's rotational time scale to less than 0.9 seconds.  It can be either positive or negative depending on the Earth's rotation.  The first leap second was inserted in 1972 and so far all leap seconds have been positive.

Who decides when to insert a leap second?  This job falls to The International Earth Rotation Service (IERS).

What does it mean to amateur astronomers?  Probably not much as far as viewing the heavens.  It does matter to astronomers (professional or otherwise) doing research requiring precise time measurements or anyone using modern electronic systems, such as electronic navigation or communication systems.  They depend increasingly on precise time and time interval.  LORAN and GPS systems are all based in the trace time of electromagnetic signals.  Radio and TV stations require precise time or they would not be able to synchronize the transmission of their programs to local audiences from coast to coast.

Bonnie J. Walters
Amateur Astronomer
California

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Last Updated:
10 January 2004
 

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