Dumb question - does polar alignment / lattitude adjustment vary Summer to Winter?

[g__day]

Trying to figure this our for a German Equatorial mount, based in Sydney.

Does Polar elevation vary from Summer to Winter to counter the Earth's seasonal tilt of its axis?

I ask as it seems my scope needs much greater elevation (than latitude 34 degrees for Sydney) tilt to keep tracking things since I moved it from a tripod to a pier as Winter approaches. Before I rule out simple operator error I'd thought I'd ask if the Earth's Axis changes, so the Sun is no longer so high in the Sky, does that mean month by month stars will riser higher and faller lower after the Winter solstice, meaning one should change the elevration of ones scope slowly as the seasons go from Summer to Spring and back?

[Astroman]

Earths axis tilt does not change over such a short period, maybe over 500 years you may notice some change but 6 months you will not.

The elevation of the SCP (your latitude) stays the same through out the whole year. And most likely through out your whole life.

Check to make sure your pier or tripod is level too using a spirit level of levelling bubble.

[GrampianStars]

No the axis does not noticably change, 1 revolution of precesion every 38,000 years.
but the magnetic deviation does change.
Here on the eastern seaboard of Aus the true north Mag. Deviation is steadily
moving east, currently around 10.5deg east,
heading towards a pole shift in our lifetime by looking at the graph over the last 100 years
wow thats exiting :thumbsup:
The sun rise is about 1 degree later every day relative to the stars

[h0ughy]

I often wondered the same question, especially when reading about fixed piers. Thanks very informative addition to the observatory planning fellas!

[JimmyH155]

There are two different types of magnetic "anomolies" The way a magnetic compass points to somewhere other than true north )(or south) is correctly called "Magnetic variation" or sometimes nowadays it is also called magnetic declination.
Magnetic "Deviation" is something completely different as I know from past experience. Whilst serving Queen and Country in the old days before gyroscopes and satellites - every time before we left on an overseas assignment, we would take our ship to a specially selected remote area of the harbour and we would "Swing the compass" What that meant was that we would line ourselves up (with help of tugs and buoys) on every position of the compass and would measure the difference between our true heading and what the compass said. The deviations would be noted on a "Deviation Card" which would be subsequently used for every change of course - along with the correction for "Variation".
Deviation is caused by the metal surrounding you on the ship and also maybe on the seabed, thats a lot! And it varies with the direction you are pointing.
As far as us astronomers are concerned, you may have a little bit of "Magnetic Deviation" due to your proximity to say your metal shed. This may cause you to set up not exactly pointing South.
To remember which direction to steer by compass, we remembered the adage "Variation East, steer least, Variation West, steer best"
Hope this lecture has been interesting.;) :thumbsup:

[rogerg]

It's not uncommon to have to tweak your polar alignment a few times throughout the year, especialy if using a wedge type setup (rather than an equatorial mount). In most amateur setups things move slightly over time and hence need fine tuning. But "much greater" sounds like quite a drastic change that I wouldn't epxect.

Roger.

[Vermin]

Could it be due to some component of the mount having a large thermal expansion coefficient? (Warmer = bigger in summer, cooler = smaller in winter, on one side of the mount causing a tilt).

Or a rising and falling water table affecting your mount's footing? (Earth drying out and contracting/drooping?).

[g__day]

Maybe, thanks. So do the Sun and Moon raise and set at the same spot on the horizon, and reach the same maxium altitude from the horizon every day of the year?

I just assumed over six months you'd see the Sun, moon and stars describe alot shallower arcs in Winter months - is this optical illusion alone?

[pluck]

A few different issues here.

First, you should be aligning with the true pole, not magnetic pole. If you are geographically aligned with the earth's rotation (the point of polar alignment), then precession is not an issue (ie, the rotational axis of the earth doesn't change relative to the earth, just to the stars (ie, the 'wobble). However, even precession is in the order of several life times, so you have little to worry about.

In reality, temperature changes tend to have little effect, unless you experience consistent ground freezing - in which case you can have enormous problems (the reason why our US based colleagues pour massive amounts of concrete below the frost line for their piers).

Achieving perfect level is not necessary for an equatorial mount - however it does help with the polar alignment process. Adjustments to one axis will affect the other with a non-level mount.

So, in short, the answer to your original question is "no".

:-)

[Dennis]

The plane of the Earth’s equator is at an angle of 23.5 deg to the plane of the Ecliptic (Earth-Sun plane).

This means (approximately and simplistically) that in the Southern Hemisphere:

Mid-Summer (21 Dec) – the Sun rises 23.5 degrees S of E (Summer Solstice)
Mid-Spring (21 Sep) – the Sun rises in the E (Spring Equinox)
Mid-Winter (21 Jun) – the Sun rises 23.5 degrees N of E (Winter Solstice)
Mid-Autumn (21 Mar) – the Sun rises in the E (Autumnal Equinox)

Based on Brisbane’s southerly latitude of 27.5 deg

At the Summer solstice the Sun is approx 86 deg above the horizon at mid day.
At the equinoxes the Sun is approx 62.5 degrees above the horizon at mid day.
At the winter solstice the Sun is approx 39 deg above the horizon at mid day.

Cheers

Dennis

PS - Solstices and Spring and Autumnal Equinoxes reversed from standard N hemisphere centric perspective.

Edit 2 - added some calcs as follows:
At Brisbane’s latitude of 27.5 deg S, the celestial equator is inclined at 62.5 deg to the horizon. Therefore;

When the Earth-Sun plane (Ecliptic) is 23.5 deg “above” the celestial equator, this puts the Sun at 62.5 + 23.5 deg above the horizon = 86 deg.
When the Earth-Sun plane (Ecliptic) is 23.5 deg “below” the celestial equator, this puts the Sun at 62.5 - 23.5 deg above the horizon = 39 deg.
Where the Ecliptic intersects the celestial equator at the two equinoxes, then the Sun will be at 62.5 degrees above the horizon.

[g__day]

WoW!

So the Sun (and presumeably all Star's) arcs do change significantly over the sesaons by the above post - but the SCP doesn't alter - its that the answer to this apparent conumdrum?

The pier is dead level on the mounting plate - both by a spirit level and the GE mount's bubble - so any error is my procedural fault I'm sure. I just have linited NW view and not to the full meridan so it makes drift alignment much more challenging for me.

[Dennis]

Hi G_day

The Sun's position on the horizon at sunrise and sunset, and it's height above the horizon when it crosses the meridian (N-S line), changes throughout the year because the apparent path of the Sun through the skies (ecliptic) is tilted at an angle of 23.5 degrees to the plane of the Earth's equator, and this is what gives us the seasons. It is purely a solar system phenomenon.

The stars are a long way away and are not affected by the plane of the Earth’s equator being tilted at 23.5 degrees to the plane of the Ecliptic, our path around the Sun.

The SCP does actually precess, or wobble like a spinning top, but it takes approx 26,000 years to make once full circle, so in our lifetime, the SCP will not really change its position on the celestial sphere.

Cheers

Dennis

[[1ponders]]

Bit more trivia about the earths motion through the skies.
Yep the earth precesses every 26,000 years (like a top), the earths axial tilt changes over 40,000 years and will swing from relatively extreme at axial tilt 24.5 deg to fairly mild at axial tilt of 22.1 deg.

[Dennis]

Whilst we are on the topic, it is worth noting that the planets do not appear just anywhere in the skies (or on the celestial sphere). Their orbits are such that their movements in our solar system are restricted to a "belt" only ± 8 degrees wide, centred on the ecliptic. This 16 degree wide belt is the zodiac and contains those signs which the planets “appear” to move through, hence their name “the Wanderers”, which means planet.

Cheers

Dennis

[g__day]

How illuminating. I always thought it was the Earth titling back and forwards through 23 degrees each year across our seasons, not the plan of our travel around our Sun. Hence I always wondered 1) shouldn't the SCP tilt over 23 degrees each Seasonal change and 2) how stable is our planet if the wobble is that much!

Don't ask - don't learn!

[middy]

To see the effect of precession open up your favourite star chart software. Set up a wide view of the east for tonight at 8 pm. Scorpius should be about 45 deg up. Increment the year by 1 a few times and verify that Scorpius is still roughly in the same position at the same time each year.

Now jump forward by 12,000 years or so. Now you should see Orion, roughly where Scorpius used to be, rising in the winter sky. I couldn't verify that Scorpius was back again a further 12,000 years on because CDC maxes out at a year of 20,000 (you will have to upgrade CDC after the year 20,000. Hopefully it will still be free :P )

[[1ponders]]

Or if you have Starry Night zoom in to get about a 40 - 50 deg wide view of the SCP (make sure you can see sigma Octanis) and run it forward using a year as a step. At some point you will see that the small magellanic cloud is only about a degree from the pole. It make for an interesting view if you complete the full 26000 years

[pluck]

The answer is that the two have nothing to do with one another. Polar alignment is simply the means by which you make your mount's polar axis parallel to the earth's rotational axis. Precession doesn't afffect polar alignment, because the whole earth wobbles - including you and your mount.

As stated earlier, levelling does not affect polar alignment. A level mount makes it 'easier' to polar align, by limiting the affect that the adjustment of one axis has on the other. However, you can polar align without a level mount.

A mount is a very dynamic system. It is possible that it is 'flexing' out of alignment in various parts of the sky - particularly if you have a heavy instrument on a small mount, loose screws, floppy optics etc. etc.

Have fun !