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Author Topic: Do we see the same stars all year round?  (Read 28028 times)

Brian Simpson

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Do we see the same stars all year round?
« on: 05/04/2011 04:30:03 »
Brian Simpson asked the Naked Scientists:
   
Dear Naked Scientists
 
I have pondered this question for many years and having just recently come across your podcasts thought you could help.
 
When I look at the night sky in January I see all the familiar star systems.  During January the earth is at a point on its movement around the sun and I am looking away from the Sun – because it is night time.
 
I have attached a sketch of my problem.
 
Six month later the Earth is on the opposite side of the Sun and at night I am looking out at what I would believe to be 180 degrees in the opposite direction to where I was viewing the night sky in January.  However I see the same star systems but only slightly tilted.
 
Why shouldn't I see a completely different set of stars than the January one as I am now opposite my original position.
 
Is it because we are really so so small.
 
Regards
 
Brian
From Griffith, New South Wales, Australia


What do you think?
« Last Edit: 05/04/2011 04:30:03 by _system »


 

Offline burning

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Do we see the same stars all year round?
« Reply #1 on: 05/04/2011 15:39:19 »
It's a little tricky to explain this without props, but I'll give it a try.

The axis of Earth to first approximation always points to the same points on the sky (there is a wobble, but it's small and slow).  That means that the rotation of Earth makes it appear that the stars circle around the celestial poles, where the celestial north or south pole is the point that would be directly overhead if you were standing on the geographic north or south pole.

Now when you live away from the geographic poles, the celestial poles are still fixed points in the sky, they just aren't directly overhead.  Through the magic of the internet, I see that Griffith, New South Wales is at about latitude 34S, which means that the south celestial pole will be in your sky at a point due south about 34 above the horizon.  All the stars in your sky will appear to rotate around this point.

That means that any star that is less than 34 away from the celestial south pole is never going to drop below the horizon where you are.  So when night comes around it is guaranteed to be in your sky, just in a different part of the sky depending on the time of year.

Stars further than 34 from the celestial south pole will drop below your horizon at some point.  The further away from the pole, the more time the star will spend hidden by the Earth.  Some parts of the year that period of time will coincide with daytime when you wouldn't see the star anyway and other parts of the year it will coincide with night meaning that the star will be lost to view.
 

Offline briligg

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Do we see the same stars all year round?
« Reply #2 on: 05/04/2011 18:10:42 »
It might help to say something about how the seasons work. People commonly think the seasons change because the axis of the earth tilts back and forth, but that isn't the case, as burning said. The earth's axis always points in the same direction. The seasons change because as the earth moves around the sun, our orientation to it changes. I once saw a great way of demonstrating this, using a ball that was one colour on top and another colour on the bottom, and a lightbulb. Hold the ball up to the light bulb and tilt the top side away from it, like how the northern hemisphere is tilted away from the sun in winter. Now, keep the tilt just the same, and move the ball to the opposite side of the light bulb. From there, the top of the ball is tilted towards the light bulb, like the northern hemisphere in summer.

Now imagine you are a flea 34 degrees from the top of the ball. As the ball spins on its axis, your view of your surroundings will change. Stick a long pin straight down towards the centre of the ball where the flea was, and imagine a line extending into space from that pin. As you rotate the ball around its axis, that line forms a cone extending outward from the ball. Anything within that cone is always visible from the flea's perspective.

And now imagine sticking a flat piece of cardboard to the ball where the flea was with a pin. Everything above that piece of cardboard would be visible to the flea. As the ball spins, some things would disappear from view, and other things would appear. If you drew a line from the ball's axis, through the pin, straight 'south', and imagined a cone shaped like the figure that line sweeps through as the ball rotates, anything within that cone would never be visible from the flea's position.

And now for the grand finale. Take the ball with the cardboard stuck to it, and tilt it like you did before, put it on one side of the light bulb, and rotate the ball like the earth rotates on its axis. The side of the ball shaded from the bulb is the side where the stars above the cardboard, marking the horizon, will be visible, when the cardboard turns into the light it is daytime and the stars can't be seen. Move the ball to the opposite side of the light bulb. Now the stars that were hidden by the daylight are visible, and the ones that were visible before are now hidden by daylight.

I may be relying a little too much on imaginary fleas, bits of cardboard, straight pins and balls, but i think this is all correct.
 

Offline Soul Surfer

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Do we see the same stars all year round?
« Reply #3 on: 05/04/2011 19:04:45 »
The reason is that you are looking towards the celestial pole and you are seeing the circumpolar stars.  This is in your case is south towards the south celestial pole with the southern cross.  If you were to turn round in the other direction and face to the north (the direction where the sun is highest during the day) you would find that the stars that you see do change regularly throughout the year.   When during your summer (December - January) you would see Orion and in the winter June and July  Scorpio and Sagittarius high in the sky with a spectacular milky way .  This is a view of the sky That I would very much like to see some day.
 

Offline mattm

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Re: Do we see the same stars all year round?
« Reply #4 on: 20/04/2014 11:50:36 »
I have thought about the same thing, and when looking at the milkyway we are looking into the center of our galaxy and during the other halve of the year we should be looking OUT of our galaxy but it seems that we see the same night sky all year round but I dont quite see this reasoning even with the tests the others have offered. I can only assume that we are not and that the sky with our naked eyes looks very similar, wish i had an awesome telescope.
I know that most of the stars are actually galaxy's very far away and should not change, but I should be looking at a different set of galaxy's, as I would assume that the shape of our own milkyway should year round. Like looking at the room wall while standing in the middle then by 6 months I should be looking at the opposite side of the same room.
 

Offline CliffordK

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Re: Do we see the same stars all year round?
« Reply #5 on: 20/04/2014 16:08:39 »
I live near the 45th parallel. 

Polaris, the North Star is visible every night in about the same place, and since it is quite distance, our trek around the sun doesn't appreciably change its place.

The rest of the stars seem to rotate around Polaris. 

Other northerly features such as the Big Dipper will also be visible every night, but starting in a different place every night for its trek around the sky depending on the seasons.

The view of stars near the equator, however, would be dependent on which side is facing the sun (not visible), or away from the sun (visible).  So you would see a different selection of stars depending on the time of year.

The planets will also come in and out of view depending on the place in their orbit with relation to Earth and the sun, and will be visible at different times of night.  Each planet has a different length of year.  The inner ones (Mercury & Venus) have a shorter year that Earth, with Mercury's year about 3 months long.  The year for the outer planets is several years long, so Jupiter's year, for example is about 11 years long.  Thus the planets will always be in a bit different place in the night sky if they are visible depending on their location and Earth's location.
 

Offline RD

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Re: Do we see the same stars all year round?
« Reply #6 on: 20/04/2014 16:27:00 »
Six month later the Earth is on the opposite side of the Sun and at night I am looking out at what I would believe to be 180 degrees in the opposite direction to where I was viewing the night sky in January.  However I see the same star systems but only slightly tilted.

The star patterns do change slightly when viewed from diametrically opposite points on Earth's orbit around the sun ...



This can be used to measure the distance of close stars via parallax.


BTW "circumpolar constellations" are visible all year round.
« Last Edit: 20/04/2014 16:34:34 by RD »
 

Offline evan_au

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Re: Do we see the same stars all year round?
« Reply #7 on: 20/04/2014 23:37:37 »
Quote
I should be looking at a different set of galaxy's
All of the individual stars you can see with the naked eye are inside our own galaxy; stars in other galaxies are too small to see individually (unless they go supernova - and that is only temporary).

Most stars in our own galaxy are also too far away to be individually visible, but the sheer number of them are seen as a brighter band across the night sky (the Milky Way). While the southern hemisphere faces the center of our galaxy, the difference is not as great as you might expect, since the plane of our galaxy is filled with dark patches of dust, which hides many stars in our galaxy.

There are a couple of nearby galaxies that are visible to the naked eye as a lighter patch in the sky - the Large & Small Magellanic Clouds in the southern hemisphere, and Andromeda in the northern hemisphere.

Quote
Why shouldn't I see a completely different set of stars?
If you lived on the equator, the stars would be totally different at the same time of night, 6 months apart.

If you lived on the equator, and compared the stars just after sunset one day, and just before dawn 6 months later, you would see that they were pretty much the same.

On the other hand, if you live at the poles, the stars are pretty much the same all year around (except it can be hard to see stars during the long polar summer).

For someone living at Griffith, the sky is a mix of stars that mostly stay the same (when looking towards the South), and stars that mostly change with the seasons (when looking towards the North).

The idea behind the horoscope is that certain equatorial constellations change with the seasons, and this is valid astronomy. Try getting a star map with the constellations marked, and follow them through the year. (In contrast, the idea that these constellations directly impact every day of your life is pure astrology.)
 

Offline acsinuk

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Re: Do we see the same stars all year round?
« Reply #8 on: 28/04/2014 10:31:27 »
I have just had an interesting thought.  Venus is inside us and rotates the sun in 224 days. So we see it as the evening star or morning star.
Does this means at the equator we see it for 4 months as an evening star and 4 months as a morning star or can we see it as both?  What happens at the poles??
CliveS
 

Offline evan_au

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Re: Do we see the same stars all year round?
« Reply #9 on: 28/04/2014 13:09:52 »
The planets (including Venus) move across the relatively fixed constellations of stars.

Venus, with its thick cover of reflective clouds over a rocky surface, is brighter than the any of the night-time stars, and is often the first thing visible after sunset, and the last thing visible before the Sun rises. But ancient astronomers could tell by its motion that it is not a star, but a planet. So the common names "morning star" and "evening star" are not accurate names.

Venus orbits the Sun in 224 days (almost 8 months). The Earth does not stand still during this time, but completes an orbit in 365 days. So a full cycle from "morning star" to "evening star" and back again takes 584 days (19 months).

From the equator (or temperate latitudes), we can see Venus alternately in the morning or evening (not both on the same day). And in between, it is lost in the glare of the Sun for perhaps a month, as it passes between Earth and the Sun, or around the other side of the Sun. (If you look closely, you can sometimes see Mercury just after sunset or just before dawn, but it is dimmer, never moves so far from the Sun as Venus does, and so is much harder to see.)

Because Venus and the Sun (and other planets) all travel around the plane of the ecliptic, at the equinox they would seem to circle around the horizon, as seen from the poles. However, the Sun is much brighter than the planets, so whenever the Sun and planets are above the horizon, the planets will be pretty much lost in the glare of the Sun.
 

Offline acsinuk

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Re: Do we see the same stars all year round?
« Reply #10 on: 05/05/2014 12:21:11 »
Thanks Evan
So at the equator we have an evening star for 4.5 months moving from west to east which after midnight changes to morning star for 4.5 months then nothing for 2 weeks. When it reappears it is an evening star which is really the planet Venus reappearing behind the sun moving from east to west for 4.5 months before becoming the morning star for 4.5 months. After 2 weeks we start that complete cycle again?
CliveS
 

Offline Michail

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Re: Do we see the same stars all year round?
« Reply #11 on: 27/05/2016 10:20:15 »
During night, earth rotates!
You do NOT just see one part of the sky during night!
You see the WHOLE sky!
The earth turns from one side, to 180 degrees in the opposite direction during night.
Turn your self 180 degrees in the opposite direction! Is there something you will not have seen around you?
So you see the whole sky during night, not "JUST ONE SIDE"...
 

Offline evan_au

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Re: Do we see the same stars all year round?
« Reply #12 on: 27/05/2016 22:44:28 »
Quote from: Michail
So you see the whole sky during night, not "JUST ONE SIDE".
Unless you live at the equator, there are some stars that never rise in your location. You will never see these unless you go on a long journey. (Even if you do live on the equator, it is hard to see stars that are near the poles, as these are too close to the horizon.)

On any given day of the year, there are some stars that are invisible because they are directly behind the Sun (the Sun is about 0.5 degrees wide, from the Earth). You will not see these stars.

On any given day of the year, there are some stars that are invisible because they are too close to the Sun, and are lost in the glow of sunset or sunrise. You will not see these stars with the naked eye, but you might be able to see bright stars if you have a computer-pointed telescope.

You may be able to see bright stars when the Sun is at least 15 degrees below the horizon (1 hour after sunset or 1 hour before dawn), but dimmer stars require the Sun to be farther below the horizon.

So you can't quite see your whole sky, but you can see most of it if you look after sunset and again before dawn.
 

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Re: Do we see the same stars all year round?
« Reply #12 on: 27/05/2016 22:44:28 »

 

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