Direction of sun rise & set

[j.skett]

Hello
New to the forum. I live in Perth and have developed an interest in telling time & direction using the sun and stars.
I have trouble getting my head around some of the basic concepts concerning the movement of the earth around the sun.
No matter how much I read in books or on the internet I still have trouble grasping why we see different constellations throughout the year, why seasons occur etc.
The thing giving me trouble understanding at the moment is why in the summer here in Perth the sun rises in the South East and sets in the South West. I know this because I have taken compass bearings of sun rise & sun set in the summer. I would have expected the sun to rise in the North East & set in the North West.
My reasoning (obviously flawed) for thinking this is because the furthest South the sun gets throughout the year is 23.5 degrees (Topic of Capricorn). As Perth is at 32 degrees South I would have thought this would make the sun to the North of us.
Can anybody help me out in trying to understand why this is so. I tend to understand better from a practical demonstration, so if anybody could suggest a way to demonstrate this I would be very thankful.
Jim

[speach]

Remember the earth is tilted at aprox. 23 deg from the plane of it's orbit. The tilt is 'Fixed' with reference to the sun, so in the southern hemisphere summer the sun , at noon is about 23 deg higher, then at noon in the winter, and so the Antarctic is in sun light in summer and darkness in winter. Now to the drift of the rising and setting sun. If the tilt wasn't there the days would be of the same length all the year but because of the tilt so we get different length days. So in summer the sun rises south more because tilt is towards the sun whereas in winter the northern hemisphere is tilted towards the sun and southern is away so the sun rises more northerly . hope this helps. If all else fails get two oranges, say, stick a pencil through one (the earth) place them apart by say 2 metres tilt one about 23 deg observe where the pencil is pointing then 180 around the central one (sun) keep the pencil pointing at the object you chose on the ceiling. you will now see the 'sun' is lower, with reference to the surface of the orange. And what was 'day' facing the sun is now 'night'. This also explains celestial progression.

[The_bluester]

Regards why you see certain constellations at certain times of the year.

Ignore for a moment the proper view of the universe and assume that the sun is the center of the universe. Take the time of year that the constellation Orion is at it's highest in the middle of the night (Coming soon) That is because we are in a position in our orbit around the sun where the earth is between Orion and the sun so Orion is overhead at midnight. At the same time, the sun is between the earth and Scorpius, so it is not visible, it is overhead at mid day.

Roll forward six months so we are on the other side of the sun in our orbit. Now we are on Scorpius' side of the sun so it is overhead at midnight and Orion is on the far side of the sun so it is overhead during the day. Obviously this happens gradually as we orbit the sun during the year.

[pluto]

Regarding why the Sun appears to rise and set to the south.
I was trying to visuallise this myself so I made a few pictures that I think help to illustrate it.

So this is just a rough Earth at the middle of a southern hemisphere summer.
The pink line is a line going from (roughly) Perth to the Sun.
The first picture is at sunrise and shows that the line lies to the south of the latitude line from the viewer.
The second pic shows sunset and shows the same.
The third pic is at roughly midday and shows that the line to the Sun is slightly to the north.

[jayeson]

Your reasoning about the northerly location is correct at the middle of the day. At the end of the day your viewing location has rotated roughly 90 degrees which is a whole other configuration.

In the middle of the day, the tilt of the earth (in summer, moves sun position southward), along with your tilt from your southerly position (you are tilted towards south, making the sun appear northwards) combine. From Perth, your tilt away from the sun is still larger than the Earth's tilt is bringing you towards it. But when the earth has rotated 90 degrees from that position, your tilt is perpendicular to the sun and has no effect.

The trick to visualizing any of this is to use models. Some folks can model 3D space effectively in their head, otherwise grab some sports balls.

Place a ball representing the sun off in the distance. To represent Earth, grab a basketball, which has conveniently marked poles, and place a dot on it to represent your location. Now put your left middle finger on the pole you decided was south and your right middle finger on the other pole. Hold your arms outstretched so Earth is between your eyes and the sun ball, left hand down, right hand up. The Earth's axis should be vertical and perpendicular to the sun - no tilt yet. The silhouette you can see around the Earth ball is experiencing sunset/sunrise. Spin the earth, left towards you until your dot comes around and is on the left silhouette. Your dot, and every point on the left side of the silhouette will have the sun ball to the west. If your dot moves north or south on the ball, the sun ball is still to the west.

Now push the south pole towards the sun and the north pole towards you. Note that from the perspective of your dot, the sun has moved southward. Something else happened too - from where your dot is, the surface of the earth started to fall away and exposed the sun more - your dot is no longer at sunset. Tilt the Earth in the opposite direction, to represent winter, and the earth rises up and blocks the dot's view of the sun - the day is already over. The Earth doesn't change it's tilt like that of course (at least not on short time frames), to more accurately represent winter, you should move to the opposite side of the sun ball without changing the tilt. The effect is the same, the north pole is towards the sun.

Edit: I didn't see Hugh's post before I started writing this, I think his pics show it clearly.

[j.skett]

Jayeson, Hugh, Paul & Simon

Many thanks for responding to my question. I have read your answers over and over trying to visualise what you are saying, however for the life of me I still can't get it. However I reckon if I keep at it, it will suddenly click.

Jim

[pluto]

If you just look at the first of my pictures and imagine that the Sun is straight to the right of the Earth. Then picture the east direction, east would be roughly towards the top right of the image.

So if you're in Perth and you're looking due east at sunrise in the middle of summer, then you're actually looking up above the orbital plane of the Earth and up above the Sun. So the Sun would be to you're right and to the south.

Hope that helps :D

[The_bluester]

Ok, to add a little more detail to my explanation that might help you.

Get a bit of paper and draw a small circle on it, call that the sun.

Out on the edges write "orion" on one side and "scorpius" on the other.

Half way between the sun and your "constellations" draw a circle around the sun to represent the earths orbit (obviously this is immensely out of scale)

Now, draw a circle on this "orbit" to represent the earth, make it directly between your sun and your orion, shade in the half that is facing away from the sun to show that that side of your earth is in darkness. The night (dark) side of your earth is facing orion, so orion is high in the sky at night and visible. The side facing scorpius is also facing the sun, so scorpius is in the sky during daylight and not visible.

Now, roll forward six months, the earth will be on the other side of the sun, now the side away from the sun (night time) can see scorpius and the side that would see orion is in sunlight.

[Robh]

Hi Jim,

From a different perspective. The Sun is not moving (ignoring its axial rotation, movement within the galaxy and the motion of the galaxy as well). It is the Earth that is moving both on its axis and around the Sun (orbital motion).

On any day, the path of the Sun in the sky is an illusion caused by the axial rotation, tilt and curvature of the Earth. In winter, the Sun is lowest at midday and this path appears shallower i.e. is not followed as far around the curvature of the Earth. In summer, the Sun is highest at midday and the path can be followed further around the curvature of the Earth.

Remember that all the constellations are in the sky all the time. It's just that in the daytime, when your side of the Earth is facing the Sun, you can't see the constellations behind the Sun. As the Earth orbits the Sun over a year, the constellations behind the Sun will shift relative to the Earth's position. In the same way, the constellations you can see at night at a particular time (say 10pm) will also shift over a year. The Earth takes roughly 365 days to orbit the Sun. There are 360 degrees in a revolution. So this shift in the night-sky view (again at say 10pm) is about 1 degree per day or 30 degrees per month or roughly one of the 12 zodiacal constellations per month.

Regards, Rob

[Renato1]

Imagine it is midday in summer. The sun is near 23.5 degrees south in declination from the celestial equator (a line in the sky that corresponds exactly with earth's equator. Facing north, to you it is almost overhead but a bit north of you. When it rises and sets, it has to be south of the east-west line.

Imagine it is midday in winter. The sun is near 23.5degrees north in declination from the celestial equator. Facing north, to you it is only around halfway up in the sky - a whole 47 degrees down from where it was in summer. When it sets, it has to be north of the east-west line.

I hope this helps with your visualization (rather than hinders). If you lived on the equator, it would be easier to visualize.
Cheers,
Renato

[Wavytone]

Peter there is a book you should buy and read - http://books.google.com.au/books/about/Practical_Astronomy_with_Your_Calcu lator.html?id=DwJfCtzaVvYC

Explains all with simple diagrams.

This is an oldie but a goodie. There are more mathematical texts (notably by Jan Meeus) but I suspect they will be beyond you.

[mithrandir]

Jim, if you have an iDevice or Android there is "Sun Seeker" (not free though - support your Aussie developer) - there might be others - which will show you the Sun's path for any date or location. The "3d View" lets you point the camera around and you can see the Sun's path against the picture.

There is also http://www.esrl.noaa.gov/gmd/grad/solcalc/ but remember to tick the "Show on map" boxes.

[SteveInNZ]

If you visualize things, this approach might work for you. I don't mean to be disingenuous if it doesn't.

Homer and the lava lamp - Or why you see different constellations.

In an average sized lounge room in Springfield, take out all the furniture except for a really bright lava lamp in the middle of the room. On one wall is a picture of Marge and on the opposite wall is a picture of Bart, Lisa and Maggie. Walking around the lava lamp, doing pirouettes, is Homer.

When Homer is on the side of the room with Marge's picture, he can see Marge when he's facing away from the lava lamp (night) but can only see the blinding lava lamp when he's facing the other direction (day).

If he keeps walking around the lava lamp until he gets to the other side of the room, he'll see the kids when he's facing away from the lava lamp and only a bright light when he's looking towards the lamp.

Slow that all down so that it takes 24 hours for planet Homer to do a pirouette and a year to walk around the solar lava lamp. At the start of the year he can see the constellation Marge and then six months later he can see the constellations Bart, Lisa and Maggie.

Hope that helps (or at least doesn't make it worse).

Steve.

[Kunama]

Now even I am confused........

[SkyWatch]

I find Stellarium is useful for this too (get a free copy at http://www.stellarium.org/ ). Once you have set it up for your location, zoom it out a bit, hit the "," button to bring up a red line for the ecliptic (the line of our solar system, which goes right through the sun), and then experiment with speeding things up: the "+" button moves it a day at a time, the "]" a week at a time, and repeated pushes on the "L" speeds it up each day ("k" brings it back to normal speed, and "j" retards it. "8" brings it to the current time.) Watch how the sun moves each time.

Alternatively, come to my portable planetarium ;) - only trouble is that I am in Adelaide! -Or check out a local Perth one.

All the best,

Dean

[j.skett]

Paul and Steve

I reckon I now understand why we see different constellations at different times of the year. Thanks for the verbal illustrations, especially the Simpson's one. It helped me get it.
Now I need to work on understanding why the Sun comes up in the SE and sets in the NW during Perth's summer. My small brain just doesn't get it.

Jim

[SteveInNZ]

D'oh !

[SteveInNZ]

Jim,
Do you understand why you can have a 24 hour day in the summer and a 24 hour night in winter in Antarctica ? Could you explain it to someone if they asked ?
Just trying to figure out where to start the answer to your question.

Steve.

[Camelopardalis]

The way I see it, and I know it's not strictly correct, is that in our summer the Sun has a more southerly declination and so whether rising, high in the sky, or setting it has to be further south than the celestial equator. As the celestial equator is the line from due east to due west, peaking at an altitude equivalent to 90 degrees minus our latitude, then the Sun must therefore rise south of east and set south of west between the September and March equinoxes. This would be reversed if you were in the northern hemisphere.

(For reference, the Sun only rises due east and sets due west on the equinox days, regardless of our position on Earth)

[j.skett]

Steve

Without researching the answer I would say that the reason why you can have a 24 hour day in the summer in the Antarctica is because when the Sun sits over the Topic of Capricorn the sun never fully sets in the Antarctica and when the Sun sits over the Topic Of Cancer the Sun never rises in the Antarctica. Sorry I can't really explain it in much more detail.

I can see from everybodys input that at the Spring Equinox the Sun rises due East and sets due West. From then on the Sun moves further South till it gets to its furthest point (Topic of Capricorn) Therefore it means the Sun must be coming up South of East in summer.

However I have this thing in my mind that persists in saying that when the Sun is at its furthest South (23.5 degrees) it is still North of Perth (32 degrees) therefore the Sun must rise North of East.

What am I missing here.

Jim

[Camelopardalis]

It's not only the Spring Equinox but also the Autumnal Equinox where that happens, and it's because of the motion of the Sun across the equator.

What your missing is that different points in the day, the angle subtended between the ecliptic and the celestial equator changes - because the Earth doesn't rotate inline with the ecliptic but an angle of 23.5 degrees - and this is most noticeable towards the solstices.

Since none of us are managing to convince you in terms you can understand, download a copy of Stellarium http://stellarium.org and see for yourself by advancing minute by minute or hour by hour until you are satisfied that is true...;)

[speach]

did you find the link I messaged to you helpful?

[AstralTraveller]

Sorry I haven't had a chance to sit down and write a reply earlier. Here's how I reason it. I hope someone will jump in if I'm mistaken.

Start with an observer on the equator on a perfectly flat plane. If the pole star had a declination of exactly +90* (* = degrees) it would sit on the horizon due north of the observer, neither rising nor setting (I'm taking it to be a mathematical point with no size and am, of course, ignoring refraction) . Likewise a hypothetical object at -90* would sit on the horizon due south of the observer. Halfway between, objects with a declination of 0* would rise due east, past directly overhead and set due west. An object at +20* would rise 20* north of east, be 20* from the zenith at it highest point (ie when it culminates) and set 20* north of west. Similarly for a star of any declination. The star at 0* would be in the sky for 12 hours and all other star for less time. I think that is fairly intuitive.

Now lets consider an observer at 30* south. For this observer, an object with declination -90* is stationary 30* above the horizon. In fact all stars south of -60* never touch the horizon, they are circumpolar (eg the Southern Cross). A star touching the horizon due south of the observer has a declination of -60*. Similarly stars north of +60* never rise and a star on the horizon due north has a declination of +60*. So the arc of 180* on the ground from due north to due south only covers 120* of declination! Now symmetry dictates that an object at 0* dec. must rise half way between +60* and -60*, which is half way between due north and due south and so is due east of the observer. No matter where you are stars with a dec. of 0* rise due east (an set due west).

Next, consider the line going from due south, through the zenith to due north. A star on that line on the northern horizon has a dec. of +60*. A star on that line 10* above the horizon has a dec. of +50* and a star 60* above the horizon has a dec. of 0*. So now the star with a dec. of 0* rises due east but appears to move to the left as it rises and culminates 30* from the zenith (and 60* from the northern horizon). A star with dec. of -30* rises south of east (see Note 1), moves to the left as it rises and passes directly overhead. A star with dec. of -50* rises in the far south and culminates 70* above the southern horizon.

Note 1: I can't work out the exact geometry but it isn't 30* south of east. If you look at Stellarium with both equatorial and azimuth grids visible you will see that the relationship between dec. and azimuth isn't linear. If I find out how to compute the azimuth exactly I'll update this post.

I hope this helps rather than hinders your understanding! :thumbsup:

[Joshua Bunn]

Hi David,

Neat post :thumbsup:

Just something that cought my eye...



All stars would be in the sky for 12 hours, their tangental velocity is just slower and so they travel less distance. Or maybe I'm misinterpreting what you said?

Josh

[AstralTraveller]

Thanks Joshua, you are right. At the equator stars rising anywhere along the horizon have the same RA and so will be in the sky for the same length of time. It's away from the equator where stars rising in different directions have different RA and so will be in the sky for different periods of time.

[j.skett]

Sorry David

Your explanation way to complicated for me. You lost me before I even finishing reading the first line.
I'm going over and over everybody's explanations hoping it will click in my head.

Jim

[SkyWatch]

Hi Jim,

Just a quick comment. I notice that earlier you said:


Is this part of your confusion, or is it a typo? The sun rise in the SE and sets in the SW (not NW) in Perth's summer...

Have you tried using Stellarium yet? If you open it, set your location and hit the "e" button. It will then show the projected latitude and longitude lines. Just imagine we are inside a big ball made out of these lines: the axis of the ball goes through the N and S poles. If you follow the line the sun is sitting on at any give date you will see how it crosses the sky, and where it rises and sets. (Remember, we are the ones in the "ball": not the sun- so as the "ball" rotates around its axis, the sun and stars stay on the same latitude line each day- and during the year the sun "moves" between the latitude lines of (approx) 23N and 23 S because our axis is tipped over as we go around the sun each year. The axis always points towards the same point of the sky as the earth moves around the sun (PS: nobody say anything about precession please: that just confuses the issue!), and the southern side of the earth is now "tipping" towards the sun for our summer, so the sun appears to be moving south to the southern latitude lines...)

Have fun!

Dean

[AstralTraveller]

So where, specifically, do you have trouble? If I know that we can go from there.

I put an hour into that post, I'd like to think you put a commensurate effort into understanding it.

[j.skett]

David
Appreciate the effort you put into your post, and I have read through it several times, however it goes over my head like a F18 Hornet, especially the continued mention of Declination.
I think the only way I will understand is to sit down with somebody and have them go through it with me using maybe a globe and a light source representing the Sun or old school with white board and marker.
Once again thanks for taking the time to try and explain it to me.
I have downloaded Stellarium and now trying to work out how to use it to help me visualise things.

Jim

[j.skett]

Dean
Yes it was a slip of the finger, meant to SW not NW.
As mentioned to David I have downloaded Stellarium and as you said hit the E button which brings up the LAT/LONG lines. I'm now trying to get my head around the rest of your instructions. Will let you know how I go.

[SkyWatch]

Hi Jim,

Substitute "latitude" for "declination", and "longitude" for "RA (right ascension)". Not exactly the same thing, but gives you the idea. Alternatively "up and down (North/South)" for "declination", and "sideways (East/West)" for RA...

Next time you are in Adelaide you can sit in my planetarium dome and I will show you! ;)

[kens]

I'd start with the notion that the sun is a long long way away and it is very very big. So the rays of light hitting earth are essentially parallel. This diagram might help.
http://solar.steinbergs.us/images/jpgs/earth-rotate-R-sun.jpg

Now draw in a spot for your location in summer (on the left) and winter (on the right) at the midday position. (The diagram is labelled for Norther Hemisphere)
Then draw in a line that traces the path of the sun through the day. It should be horizontal.

Now imagine a map overlaid (or maybe use Google earth) and see where the path of the sun intersects the day/night boundary.
You should see that the intersection point is at SE/SW in summer relative to your location.

It also shows why there are 24 hour days and nights at the poles

Disclaimer: I'm making this up as I go and I get confused about it all too :)

[AstralTraveller]

Ah ha. Now we're getting somewhere. You need to start by understanding the basics of positional astronomy. That will stand you in good stead, not just for this question but for many other uses (finding objects, working out when they are visible etc). Years ago I wrote the 'Sky this Month' for my local club. It was pre-computer and so everything was done manually - tables of siderial time from Nortons Star Atlas, semi-diurnal arcs from the ASNSW Yearbook (itself produced manually by the great Ed Lumley) and poring over the Skalnate-Pleso Star Atlas. That really banged the concepts into my boney skull.




Yes, RA and Dec are analogous to longitude and latitude (respectively) but they are not the same. One applies to the surface of the Earth and the other to the celestial sphere. If you don't separate the two you will soon confuse yourself. If you want to know whether an object of a given declination is visible from your latitude it will confuse the hell out of you if you start thinking 'what latitude is visible from my latitude?'

I suppose you understand this but, for the purposes of positional astronomy, we are on a sphere inside a sphere. A lot of this discussion revolves around what part of the outer (celestial) sphere is visible from our position on the inner sphere (the Earth). And, if the lines of RA and Dec were painted across he sky how would they appear and how does that change as we move to a different latitude.

Don't give up, if I could eventually understand it, so will you!

[j.skett]

Dean
Thanks for the encouragement. Yes I am confident if I keep at it, it will eventually click until then I remain frustrated that I can't grasp it.
Jim

[Paul Haese]

Just to complicate things a little or maybe help.

The reason for the apparent rise of the sun in the SE and the SW in summer is due to the axial tilt of the Earth. See the first image. This is set up for the Northern hemisphere but if you imagine that the S and N are switched that will explain things for you. We only see part of the ecliptic and that is the path the Sun appears to follow, remember we are orbiting the Sun, but the apparent view is that the sun is orbiting us. In the top part of the image the dotted line is the ecliptic as traced onto the celestial sphere. We are observing that path from essentially a flat plane. Even though the earth is curved it gives the apparent view of being flat because of its size and our size.

In the bottom part of the image the ecliptic is traced again by the dotted line. As you can see it again appears to move from South to North (remember I said to swap the S and N around).


The second image demonstrates the tilt of the earths axis and how the light rays are essentially parallel.

Lines of declination are the like the lines of latitude as drawn on the earth but are extended outward into space. Lines of latitude are drawn parallel from the equator (0 degrees) down to the poles (90 degrees).

As to why we see different constellations over the entire year. Imagine you are on a merry go round which is slowly turning. Looking out as it travels around. Now imagine that the circular path you are taking on the merry go round is the Earth's orbit around the sun. You will note that the view on the merry go round shows something different all the time until you are back to the starting point. Hence why we see different constellations. We are on a giant merry go round.

Did that help?