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
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.
Last edited by speach; 10-10-2014 at 05:23 PM.
Reason: More detail
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.
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.
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.
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.
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.
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.
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.
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
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, 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.
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).
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.
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,
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.
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)
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.
- only trouble is that I am in Adelaide! -Or check out a local Perth one.
