My understanding is that viewed from the Pole Star (North Star, Polaris) the orbit of the Earth around the Sun is anti-clockwise.
Likewise, viewed form the Pole Star, the rotation of the Earth about its axis is also anti-clockwise.
Many thanks for this statement Matthew. I had mis-read Mike’s post and had in my mind that he was asking which spot on the Earth would be closest to the Sun, when the Earth in general was closest to the Sun. The Earth is closest to the Sun on 4th January (usually). The longest day in New Zealand (where Mike is based) is, as you say, 21st December.
My guess is, that the correct spot on the 21st Dec will not be the same as the correct spot on the 4th January. But perhaps they are similar ?
1 Like
As the Earth moves closer to the Sun until 4 January, then I think we need the land point that has the Sun overhead at the latest time on 21 December.
Maybe Cook Islands? I think they are just east of the International Date Line.
@jrhardee made a couple of useful points. Firstly, to simplify the problem by considering a non-rotating object. Secondly, to visualise what’s happening. Here are two rockets (red and green), travelling clockwise around a fixed point, which we are observing from afar:
Q1: From our vantage point, which colour of rocket do we see as non-rotating?
Q2: If each rocket has an attached device for measuring rotation, which rocket would record some rotation? Assume that both rockets are in stable orbit, not using any rocket power.
Hopefully we can agree on the answer to Q1. However, Q2 is likely to be more tricky, especially as few of us will have had any astronautic experience! But Q2 is important if we want to resolve why there are valid differing answers to the original teaser.
I probably got my question slightly wrong, as I had incorrectly assumed that the earth was closest to the sun in the southern hemisphere on the longest day. Alas, I actually meant where on earth (in the southern hemisphere) would you need to be, to be as close as you will get to the sun. But, then I still forgot about elevation….and then the equivalent location in the northern hemisphere might be closest still….
1 Like
Q1 Green
Q2 Green
I have a different answer from @Pete_the_painter.
There are only four possible combinations:
Green, Green
Green, Red
Red, Green
Red, Red
Any further contributions from anyone?
Well Ravvie,
… a bit more thought…
To my mind,
Both rockets are Orbiting the fixed point.
The Green Rocket would make 1 x Rotation, Clock-wise, around its Normal axis, during that Orbit.
The Red Rocket wound not Rotate around its Normal axis. during that Orbit.
I have highlighted some of my terminology to try to clarify my use of words including Orbit, Revolve, Rotate, Spin etc.
Others may well have differing views …
What do you conclude in terms of the four options?
I deliberately avoided opting for GG; GR; RG or RR because I wasn’t certain about some of the terminology in your description. (I never was a very good reader !)
However, I think that the best of those four options that I would associate with my description would be:
Q1 - Red (it doesn’t rotate about its Normal axis)
Q2 - Green ( one rotation, clockwise about its Normal axis)
Hi Ravvie,
forgive me asking this, but as a non-native speaker I’m not sure if I have gotten your simplified example right: we are looking at the earth from afar and the earth isn’t rotating?
If that’s the case my thoughts are:
Q1: green, as we observe a rotation of the rocket around its center.
Q2: red, as the rocket changes its orientation relative to the earth.
1 Like
My answer is Q1: Red (appears to us as not rotating), Q2: Red (actually rotating). That means each possible answer has been picked once by the four of us! Also I think @jrhardee was inferring Red for Q1, but that was before I asked this variation of the question.
From afar (and assuming that everything other than the rockets is fixed in relation to the stars), we observe that the green rocket is rotating clockwise and that the red rocket is not rotating.
In reality, the green rocket is not rotating. The red rocket is rotating anti-clockwise once for every completed clockwise lap. It will have rotational energy (in addition to its kinetic energy of its orbit) which can be detected by a suitably sensitive instrument.
Note in comparison that the Earth’s moon always has the same face to Earth, hence is equivalent to the green rocket. The moon used to rotate, billions of years ago but is now tidally locked, meaning that tidal forces acting on the moon have brought it to a rotational standstill. Yet an external observer would see it as rotating once per orbit of the Earth.
Back to the original teaser, we can make two correct statements:
- To an external fixed observer the Earth appears to rotate about 366.25 times in a year.
- The Earth actually rotates about 365.25 times in a year.
The apparent contradiction of observed vs reality is due to us seeing space as three dimensional, but in practice space is distorted by what we perceive as gravity.
2 Likes
I still think this phenomenon requires a bit more sharing of thoughts … 
In Ravvie’s Rocket example, I had visualized each rocket (Red and Green) freely “perched” on a circular shaft, slotted into a matching circular cylinder built into the rocket. In other words, a simple bearing allowing the rocket free to rotate around its axis of rotation. And very simple to detect and measure rotation of the rocket around its normal axis.
During each orbit of the fixed point, I visualised that the circular shaft retained its N/S, E/W orientation as observed from afar. On this basis my perception was that :
-
The Red rocket in Ravvie’s diagram, retained it’s N/S, E/W orientation. Hence no rotation between the circular shaft (axis) and the circular cylinder built into the rocket.
-
The Green rocket in Ravvie’s diagram, rotated 90 deg clockwise around its shaft as the Green rocket moved 90 deg clockwise in its orbit around the fixed point. That is one x 360 deg clockwise rotation for each orbit around the fixed point.
… sharing my thoughts …
I’m not yet convinced that we are in the same church, never mind singing from the same hymn sheet
1 Like
Sticking my neck out a bit here … but …
After a bit more thought … I went back to the original question as set by Matthew which was :
“On the topic of celestal bodies, how many times does the earth rotate during one orbit of the sun?”
My (current) answer is …
Relative to the Sun, 365.25 times (approx)
Relative to all the other Stars, 366.25 (approx)
I’ve got my hymn sheets ready for distribution
1 Like
I would say that both of your statements are correct.
My physics knowledge is very rusty (47 years ago), so I might not be using the correct terminology, but the reference axis for an orbiting body is along its gravitational field lines which are at right angles to its orbit. Hence “relative to the Sun” is the correct frame of reference for the Earth. In my answer (to Q2), this is the reality for the Earth, whereas “relative to all the other stars” the observation is different.
1 Like
We seem to be “poles apart”
… and not just you and me, Mulberry … just about all of us !!
2 Likes
As Newton once said, “Gravity Sucks !!”
A dart is launched vertically upwards at a speed of 16m/s from a platform 8m above the ground, and travels freely under gravity, ie the dart is not self-powered.
At the same time, a small rock is dropped from rest at a point 40m above the ground. It also travels freely under gravity.
How long after their release will the dart and the rock both be the same distance above the ground ?
(Assume that air resistance is negligible).
Back to School !
Fourteen of the children in the class are girls. Eight of the children wear red shirts. Two of the children are neither girls nor wear a red shirt. Five of the children are girls who wear red shirts. How many children are in the class?
I’m sure there are much easier methods than my clunky use of Newtons Laws of Motion equations, but I think the total time elapsed is 2 seconds.
1 Like
I think the total number of children is the same number as a famous Paul Hardcastle Number One from 1985.
1 Like