¨ What would happen
if the sun disappeared for an hour?
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from Phil Plait on 16 November 2003:
from Jane Luu on 15 August 2003:
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.
from Mark Marley on 15 August 2003:
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!
from Stephanie Wong on 8 May 2003:
(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.
from Stephanie Wong on 21 March 2003:
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
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.
from Wendy Wooten on 6 February 2003:
from Roger Herzler on 4 February 2003:
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.
Clear Sky Clock Homepage
from Dave McCarter on 2 November 2003:
from Roger Herzler on 22 January 2003:
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)
Ursa Major (the actual
constellation the Big Dipper is in)
from Bonnie J. Walters on 26 January 2003:
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
from Bonnie Walters on 21 November 2002:
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
from Bonnie Walters on 7 October 2002:
Bonnie J. Walters
from Roger Herzler on 11 December 2002:
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.
from Bonnie Walters on 30 July 2002:
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
10 January 2004
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