We’re always exploring, always learning, human beings are inquisitive in nature, life is inquisitive, pushing boundaries, ever evolving. Its not just you and me, its the money plant in your kitchen trying to see if it can sneak in around that corner and grow; its the squirrel inspecting everything it can carry to see if its edible or not; the dog tilting its head at you yelling at him to ‘bring it back’ in some alien language. We’re curious by nature and thank god for that, for those homo erectus who migrated and spread – somehow somewhere developed the sapien part, tinkered with elemental forces, set off that wheel in motion and kindled the fire of curiousity in all of us.

The same curiousity made us humans look around and come up with an explanation for change of seasons and wind, eventhough it involved fascinating stories about angry gods, but we came up with some explanation or the other that would appease to our logic or emotional reasoning at the time and helped us made sense of these vast celestial forces beyond our control.

It’s the same curiousity that left us open mouthed in primary grade when they told us earth was not like a ball but more like an egg (well that’s how I remember I was informed about this world shattering concept at the time), because like the wise Ptolemy, all of us little humans were fascinated by the beauty and the perfect symmetry of circles, an egg or a flattened football certainly broke that notion.

Pass a few years and the well established understanding is, earth is an ellipsoid (a flattened sphere – bulging at the centre), and its revolving around the sun in an elliptical orbit and is slightly tilted on its axis. The surprises and revelations kept on coming as we learnt more and more and still do, for instance earth is a wobbly spinning top, with four kinds of motion.
The revolution around the sun,
the rotation (or the spinning motion) around its own axis,
the precession (the wobbly top movement which leads to a periodic rotation of its axis – basically after 26000 years the tilt of the earth will rotate a bit)
and nutation (another consequence of the wobbly motion – while precession rotates the axis in a conical way – this is more like nodding side to side while following that cone)

Elliptical orbit

Now let’s touch the elliptical orbit first, unlike circles with one central point, ellipses are basically elongated circles which therefore have two focii.
Where a circle is defined as a closed planar figure whose boundary is a collection of all the points equidistant from a fixed point (center of a circle, and the term for equidistance is radius), an ellipse is defined as a closed planar figure whose boundary is a collection of all the points which are ata constant sum of distances from the two foci. Figure 1 below shows this

Fig 1 – Circle left, ellipse right

When the earth orbits the sun in an elliptical orbit, sun is at one of these foci:

Fig 2 – Earth’s orbit

Now another interesting detail is technically earth is orbiting center of mass of earth and sun system – but because the earth, even though heavy enough to hold all of us – is quite small compared to the sun – the center of mass of this system actually lies within the sun and thats what is at the foci. This center of mass is not the center of sun – thats what causes this elliptical orbit. So, this visualization in Fig 2 might be a bit more elongated than the actual picture. It is actually less elongated than that and sun is almost at center – and also rotating around center of mass of earth sun system (which to reiterate – lies within the sun)

Seasons

Okay that established, you would think, there’re times earth is closer to the sun and that’s why you observe different seasons, but you’d be wrong once again, the seasonality we experience is not because of distance between earth and the sun but the tilt of the earth.

Fig 3 – Seasons

As you can see – the earth is tilted on its axis, by an angle of 23.5 degrees (the earth’s precession changes that angle – it ranges from 22.1 to 24.5 degrees – and the whole cycle takes about 40k years).
Now when earth’s northern hemisphere is tilted towards the sun, it receives more direct sun rays and experiences summer weather whereas southern hemisphere pushed away from the sun experiences winter (this is shown in earth’s positioning on the leftmost part of Figure 3 – which also depicts the position of solstice and equinoxes for the northern hemisphere as well). The same is true for the reverse, equinoxes on the other hand (equal day and night) are observed when sunrays fall at about 90 degrees to earth’s equator.

Fig 5 – Earth’s tilt and season

Ecliptic

Now since we covered seasons, lets also briefly cover the topic of ecliptic, in the previous article of Navigating the stars, we discussed celestial sphere – which looked like this:

Fig 6 – Celestial Sphere

So you can expect what’s coming next – how does the sun look like on celestial sphere – the answer is pretty normal other than appearing so big because of its closeness to us – we make no special adjustments for the sun – but since its so close and we have studied its motion for centuries, it helps us with our navigation quite a bit.

The path sun follows on the celestial sphere is called an ecliptic – and since our solar system is more or less flat (with about 8 degree deviations from a planara structure – more about that later) – most of the planets lie around the ecliptic as well – and why its called an ecliptic – you know already now.

Fig 7 – Ecliptic

Figure 7 above shows us how sun and its orbit (the ecliptic) would look like as seen from earth – and how would it be projected on to the celestial sphere. Since the celestial north coincides with earth’s north pole – our sphere here is straigh – so the sun’s path will appear tilted – at 23.5 degrees (this value will – and has changed with precession)

You might also be familiar with all the constellations along the ecliptic, our ancestors used this knowledge for time keeping and season tracking. These are the infamous zodiac signs (well zodiac signs were named after these famous constellations – because we’d see them throughout the year as we orbit the sun)

Fig 8 – Ecliptic

Figure 8 is my attempt to draw them again from an eagle eye perspective (well a spacecraft eye) – at how we see different constellations at specific positions in the sky at different months. And just to be clear, at earth we will be able to see constellations only at night so we will be seeing the ones away from the sun. For example in January, sun will be in the constellation of Capricorn – so it’ll be invisible to us but cancer is going to be the one we will see rising in the night sky.