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posted on January 3rd, 2014 at 08:52 AM
Happy Perihelion everybody!
Happy Perihelion on January 3rd.
Today is the day when the Earth is closest to the sun (no one in South East Queensland will disagree with that).
It is also when the earth is travelling at its fastest.
To get the absolute maximum speed from your VW drive due East at midnight tonight! That way you can travel at the orbital speed of the Earth plus the
rotational speed of the Earth plus 100km/h (because none of us would ever speed)!
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posted on January 3rd, 2014 at 09:26 AM
Its not helping much down central vic. Its freezing!
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posted on January 3rd, 2014 at 07:42 PM
Quote:
Originally
posted by Snap Crackle Bang
To get the absolute maximum speed from your VW drive due East at midnight tonight! That way you can travel at the orbital speed of the Earth plus the
rotational speed of the Earth plus 100km/h ...
So if I drive due West at 100 km/h, do I get anywhere?
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posted on January 4th, 2014 at 11:32 AM
If you travelled West at the equator your speed would be 109080km/h from the orbital speed plus 1670km/h from the rotation minus 100km/h.That is
110650 km/h in total!
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posted on January 4th, 2014 at 08:59 PM
110650 in reverse?
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posted on January 6th, 2014 at 06:43 AM
You were a day early! 2014 perihelion was actually on 4 January, at around 12:00 UTC (add 11 hours for Eastern daylight saving time for Sydney and
Melbourne).
Love the way you've calculated the Earth's orbital speed and rotational velocities (relative to the background stars). What about the orbital speed
of the entire solar system around the galactic core - some 220 km per second (792,000 km/h)?
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posted on January 6th, 2014 at 10:39 AM
what do you mean relative to the background stars?
I would think that would be angular velocity relative to the sun as the solar system is moving at incredible speeds relative to other stuff around it.
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posted on January 6th, 2014 at 11:41 AM
Imagine the Sun and the Earth alone. The Earth orbits the Sun. But how do you measure the orbital period? You need a start and an end reference point
to make your measurements. You can't use a point on the Sun, as the Sun also rotates on its own axis.
So you measure the Sun-Earth system at a point relative to the background stars, which do not move over the short period we are measuring. This is
called a Sidereal year (from the Latin word for star). From Earth, we measure it from the Sun's position relative to the background stars, and the
time it takes to do a complete circle of the sky and back to the starting point.
Likewise we can measure the Earth's rotation period relative to the background stars - the time it takes for one star to rise, set and return to the
exact same spot. This is a Sidereal Day.
But to add to the complexity, a Solar day is the time it takes for the Sun to rise, set and return to the exact same spot in the sky. But in fact it
is about 4 minutes longer than a sidereal day, because the Earth has also moved in its orbit around the sun during the day. This means the sun has
moved in the sky slightly since yesterday, so the Earth needs to rotate that little more to 'catch up' that distance.
You might be thinking of measuring a year from one summer mid-point to the next - or solstice to solstice. This is called a tropical year. If the
earth's axis was fixed in space, it would be exactly the same as a sidereal year. But because the Earth's axis is tilted (at 23 deg), and the axis
wobbles in a 26,000 year circle called the procession of the equinoxes, the solstices move very slightly. A tropical year is about 20 minutes shorter
than a sidereal year.
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posted on January 6th, 2014 at 01:17 PM
which star do they use for the reference in that case? They are all moving all the time. In fact astronomers have been making videos of objects like
stars moving from periods of days to several years. They have been doing this for decades if not longer.
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posted on January 6th, 2014 at 01:30 PM
to make it more fun, the earth's orbit is never the same period or path from one year to another.
gravitational and tidal forces from other planets, near by starts and even things like debris, comets and asteroids effect the period of the orbit.
for example if the earth is nudged slightly in towards the sun, the orbit speeds up due to the conservation of angular momentum. Likewise it slows
down when moved further out.
Also same is true if the earth is slowed down it will fall into a lower orbit around the sun and vise versa.
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posted on January 6th, 2014 at 02:59 PM
You could use ANY star. The Voyager space probes and the Apollo moonships normally used Canopus, the second-brightest star in the sky some 300 light
years away, with Alpha Centauri as a backup.
Yes all the stars are moving. If you could look at the night sky 1 million years ago, it would be unrecogniseable. BUT - they are moving very very
slowly and don't affect our calculations of the earth's orbit. The stars with the largest 'proper motion' are all tiny red dwarf stars that are
very close to us, but invisible to the naked eye. The largest proper motion star is Barnard's Star, which moves 10.3 arc seconds per year. The moon
averages around 30 arc minutes in apparent size, so Barnard's Star would take 174 years to move the width of the moon. Kapetyn's Star, the second
fastest moving star, moves at 5.7 arc seconds per year. You can take movies of these stars moving, but only time-lapse ones.
Alpha Centauri is closer and much brighter, the brightest high motion star, but it moves at just 3.7 arc seconds per year. It is moving in three
dimensions of course, one of which is closer to us on an overtaking path, and over the next 30,000 years will move across the top of the Southern
Cross and into the Hydra constellation. It will pass us by just 3.2 light years away (4.3 now). Then it will move away, slowing moving across the
southern sky before disappearing forever after about 100,000 years.
The brightest star, Sirius, is 8.2 light years away and moves at 1.3 arc seconds per year - 1,384 years to move the width of the moon. Brighter stars
further away are much slower. Vega (25 light years) moves at 0.34 arc sec/year; Archenar (140 ly) at 0.09 arc sec/year; Canopus (310 ly) moves 0.03
arc sec/yr. Giant star Betelgeuse (640 ly) moves at 0.02 arc sec/year; Rigel (860 ly) at just 0.001 arc sec/year.
The only bodies that affect the Earth's orbit in an orbservable way are the Sun and moon. The other planets, especially Venus, Mars and Jupiter (at
coonjunction) may have a real but unmeasurably small effect. Asteroids and comets - nope. You could calculate it, of course, as NASA used the earth's
gravity on the Galileo and Cassini probes to slingshot them outwards. They worked out that the Earth's rotation would have been slowed by a billionth
of a second over millions of years - small enough to ignore.
Orbital mechanics are sometimes counter-intuitive. For example, suppose you're in a circular orbit around the earth. You fire your engines (in the
direction of the orbit) to speed up. Do you move to a 'higher' orbit? No! You will return to exactly the same point on the next orbit but the rest
of your orbit has changed to an elipse with a higher point at the opposite end. To move to a higher circular orbit, you need a second burn at the
opposite point. Same with moving to a lower orbit. Also once you are in your higher orbit, you are now moving slower (Kepler's Law). Here's another
- Suppose two spacecraft are in the same circular orbit and wish to dock. Unless they are very close, the trailing ship cannot simply fire its engines
to go faster. This will change the shape of its orbit, causing it to gain altitude and miss its target. It needs to thrust opposite to the direction
of motion, and then thrust again to re-circularize the orbit at a lower altitude. Because lower orbits are faster than higher orbits, the trailing
craft will begin to catch up. A third firing at the right time will put the trailing craft in an elliptical orbit which will intersect the path of the
leading craft, approaching from below.
There is nothing that can slow the Earth down (or speed it up) on its journey around the sun. The orbit has been stable for 4 billion years. Only the
earth-moon system is slowly changing, the earth's spin gradually slowing and the moon moving further away (a few cm per year) to conserve angular
momentum. Earth hits by asteroids, or even large earthquakes, only change the rotaional period (and moon relationship), not the orbit around the sun.
