There are four seasons in a year: Spring, Summer, Autumn and Winter. Temperature is very high in summer, low in winter, but spring and autumn seem to have an optimum comfortable temperature. Why?

The seasons change because of the tilt of earth’s axis and its revolution around the sun. As the earth goes around the sun, the times when the axis is most tilted toward or away from the sun are called solstices and the times when the axis is tilted neither toward nor away from the sun are called the equinoxes. It is either spring or autumn around the times of the equinoxes, and summer or winter during the solstices, as you see above.

The following video made using this simulator tool helps explain in more detail.

Because earth’s rotational axis is tilted, the two hemispheres are not equally illuminated during different seasons and, consequently, the sunbeams do not fall on a location on earth at the same angle. Let us see how the sunbeam angle and the illuminated area change during different seasons in the northern hemisphere, specifically at a latitude of 24 degrees north.

1. 21 June, summer solstice: the sunbeam falls vertically down on an observer in the northern hemisphere. As the earth moves toward its autumn position, the tilt or angle of the sunbeams increases.
2. 23 September, autumn equinox: the sunbeam falls at an intermediate angle, the temperature is moderate.
3. 22 December, winter solstice: the tilt of the sunbeam is at its largest, sunbeams fall at an extreme angle, its is cold in the north.
4. 21 March, spring equinox: the angle of the sunbeam is the same as during the autumn equinox.

Why is this happening? Because the earth’s axis is tilted by 23 degrees. Because of this tilt, we are inclined more toward the sun during June, and more away from the sun during December. So the angle of the sunbeams is maximum during summer (Jun) and minimum during winter (Dec). During autumn (Sep) and spring (Mar), the angle and the temperature are medium.

But the scenario is exactly opposite for people in the southern hemisphere. They are inclined more toward the sun during December and more away from the sun during June. The southern story goes like this:

1. 21 Jun, winter solstice: sunbeam has the largest angle.
2. 23 Sep, spring equinox: sunbeam has a moderate angle.
3. 22 Dec, summer solstice: sunbeam has the smallest angle.
4. 21 Mar, autumn equinox: sunbeam has a moderate angle.

You also see that the two hemispheres are differently illuminated during different seasons. During northern summer, it is always day for 6 months in the north pole while it is always night in the south pole. During northern winter, is is always night for 6 months in the north pole while it is always day in the south pole. You see the connection between summer and day and winter and night.

We can also think of 4 seasons of a day: noon, evening, night, morning which are similar to the feelings of summer, autumn, winter and spring, respectively.

People knew about the seasons since thousands of years ago. The question is, how could they explain it if they believed the earth to be at rest at the center of the universe? It turns out you can explain the same phenomenon using a model of the solar system where the earth is at the center and the sun is orbiting the earth at an angle, that is the orbital plane of the sun has an angle of 23 degrees with the equatorial plane of the earth. See the second part of the video for a visual understanding.

Another celestial phenomenon very familiar to people of all cultures is the changing phase of the moon. Use the Islamic calendar for example. On the first day of Ramadan, the moon is narrow and it gets bigger as the month progresses. The moon is half full around the 7th day of Ramadan and a full moon appears on the 14th or 15th. Then the moon starts getting smaller. It is again half full around the 21st, and almost completely disappears at the end of the month on 29th or 30th. When a new moon rises again, Muslims observe the Eid-ul-Fitr after fasting with the moon for a month.

Do you know that the very word ‘month’ came from the word ‘moon’ in English? Also why does the English ‘moon’ or ‘month’ sounds similar to the Bangla ‘মান’? Because ‘moon’ came from a word that meant ‘to measure’ similar to ‘মান’ in Bangla.

The phases can be explained using this diagram. The moon orbits around the earth on the same plane as the orbital plane of earth around the sun. In this figure, the sun is not shown, but its light coming from the right.

1. New moon: moon is on the same side of earth as the sun (is in conjunction), we cannot see the moon at night because moon rises with the sun and sets with the sun.
2. Waxing crescent (7 days): moon appears like a crescent and increases in size, moon rises 50 minutes later every night.
3. First quarter: the earth-moon line has an angle of 90 degrees with the earth-sun line, we see half of the moon illuminated by the sun, the moon rises at midnight.
4. Waxing gibbous: the moon is gibbous, i. e. more than half full, keeps rising 50 minutes later every night.
5. Full moon: moon is on the other side of earth compared to the sun (is in opposition), rises at sunset and sets during sunrise, so we see it the whole night fully illuminated by the sun.
6. Waning gibbous: the gibbous phase keeps waning, the moon keeps rising 50 minutes later.
7. Last quarter: half full moon rises at noon, we see it in the evening when the light from the sun is not that bright, see it more clearly after the sun sets.
8. Waning crescent: the moon keeps shrinking and rises 50 minutes later until the new moon when it will rise only at sunset.

Three questions remain. First, the sun always illuminates half of the moon, so why do we see the changing phases? The phases change for us because we can see the moon only from the night-side of earth and the line of sight from the night-side to the moon keeps changing as described above.

Second question: during a full moon, why the moon is not completely blocked from sunlight by earth? if the earth is between the sun and the moon, shouldn’t the moon be completely in the shadow of earth if they are really aligned? Yes, but the fact is the plane of moon’s orbit is not exactly aligned with the orbital plane of earth, they have an angle of 5 degrees as shown below.

Credit: https://www.virtualtelescope.eu.

On rare occasions, the sun, the earth and the moon really align creating a lunar eclipse as we will see in the next section.

Third question, does the moon also rotates around its axis as the earth does? Yes it does. But for the moon, the time it takes to revolve around the earth once and the time it takes to rotate on its axis once are one and the same. So we always see the same face of the moon. The ‘dark side’ or the ‘far side’ were not known to us before we sent spacecrafts to probe that side.

You will not understand the phases properly without a 3D model. The video above made using UNL NAAP Labs does not really add another spatial dimension, but at least adds the time dimension to increase your understanding.

As the moon revolves around earth, you see that illuminated part visible from the night-side of earth is changing the crescent to the gibbous phases. You might think the illuminated part of the moon is not visible at all from the night-side of earth sometimes. But this is not the case. The rotation of earth is not shown here. As the earth rotates the illuminated part of the moon become visible at least during some parts of the night.

Earthshine: during the crescent phases of the moon, the sunlight reflected from earth illuminates the dark part of the moon. This is called earthshine, first described by Galileo in 1610. If you are in a dark location, you can sometimes see the dimly illuminated full moon along with a bright crescent. The crescent is caused by direct sunlight and the the remaining part by the sunlight reflected from earth.

Lunar cycle: If you have not thought about the influence of the moon on us, just think about the menstrual cycle in women¹⁾ and the sleep cycle common to both men and women. People sleep less during full moon, more during new moon. The moon takes around 27.5 days to go around the earth once but the phases complete one cycle in around 29.5 days. There is a 2-day difference because by the time the moon completes one orbit, the earth moves forward in its orbit around the sun. So the moon takes 29.5 days to come back to its initial position with respect to the sun.

On the rare occasions when the sun, the earth and the moon completely align during a full moon, we get lunar eclipse, that is we see the shadow of the earth on the surface of the full moon, and the moon gets darker than usual.

The orbits of the earth (blue) and the moon (green) are shown here. We can define two regions of shadow, the umbra and the penumbra, with respect to the sun. If the moon enters the umbra, it is completely under erath’s shadow and we get a total lunar eclipse. We only see a partial eclipse when the moon is in the penumbra.

Duration: a total solar eclipse can last at most 7 minutes, usually only a minute, but a total lunar eclipse can last even an hour. The reason has to do with distance. The moon is much closer to earth, so it takes long for it move out of earth’s shadow.

Earth’s shadow on the moon is always circular. During a lunar eclipse, we see a circle encroaching the surface of the moon. Aristotle proved that the earth is spherical using this observation.

The total lunar eclipse of 8 Nov 2022 shows how the moon actually looks during an eclipse. The conditions of the moon is shown from 8:55 pm to 12:14 am. At 9:44 pm, half of the moon is in penumbra, the other half in umbra. The part inside the umbra is reddish? When the whole moon enters the umbral shadow of earth, it is completely reddish. Why?

The reason is the same as the reason behind seeing a blue sky. Sunlight is white, it has all the 7 colors of rainbow, but blue light is disturbed by the atmosphere of earth more than red light. When the sun is up, the red light reaches earth’s surface without much interruption but the blue light is scattered throughout the sky by the molecules of air; so the sky is blue.

During a total lunar eclipse, sunlight is not completely blocked, some light from the sun refract (bend) due to earth’s atmosphere and then shines on the moon. As the atmosphere scatters the blue light away, only the redder lights can go to the moon uninterrupted. This is why the moon looks reddish during an eclipse and called the ‘blood moon’ giving an eerie feeling. See the timelapse video of a lunar eclipse of 2018.

Question based on the video: why does the moon turns red only after it is completely eclipsed by earth?

The blood moon of lunar eclipse and the black sun of the solar eclipse had a huge influence on the psychology of human cultures. Here is the Bible: “And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood; And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind.” — Book of Revelation, New Testament.

Sun goes completely black during a total solar eclipse, when the moon is between the earth and the sun during a new moon with perfect alignment of the three celestial objects.

Again you can see the umbra and penumbra created by the sun and the moon on earth. Earth’s shadow falls on the moon during a lunar eclipse, but there is no shadow in a solar eclipse, the very source of light is completely blocked by a piece of grey rock, the moon.

There is another big difference between the lunar and solar eclipses. A total lunar eclipse can be seen from many places on earth for almost an hour, but a total solar eclipse can only be seen within a 300-km wide patch on the surface of earth and only for a few minutes even from there; you have to be within the patch to see the total eclipse. This is basically the umbral ‘shadow’ of the moon on the earth.

You can find the time, duration and locations of the solar eclipses for the next 10 years here: https://www.timeanddate.com/eclipse/list.html. This website has maps of all total and ‘annular’ solar eclipses within 5000 years from 2000 BCE to 3000 CE: https://eclipse.gsfc.nasa.gov/SEpubs/5MCSE.html. What is an ‘annular eclipse’ will be clear shortly.

Let us first see the video of a total solar eclipse. The moon encroaches on the sun and we see the a circular disk sliding on top of the sun. As the sun

You also see the shapes of the constellations. A constellation is a group of stars that form a recognizable pattern when viewed from the earth. These patterns were used for navigation by our ancestors and gave rise to numerous myths throughout human history. The patterns are of course imaginary. Some of the stars in a constellation might be very close to us, while some are extremely distant. As the surface of the earth is divided into many countries and oceans, the surface of the imaginary celestial sphere is divided into 88 constellations.

We identify people using their countries. Similarly, we identify celestial objects using the constellations in which they belong. For example, the Great Orion Nebula belongs to the Orion constellation. And here is a map of the Orion Constellation, the country of Orion.

You see the borders and also its neighboring constellations; this is how we would show the map of Bangladesh as well with India and Myanmar as neighbors. Three stars (Alnitak, Alnilam, Mintaka) constitute the belt of the Orion. Betelgeuse and Bellatrix are the hands, Saiph and Rigel are the legs. Our ancestors devised a way to remember this region of the sky using the shape of Orion, a mythic hunter.

Use https://stellarium-web.org for exploring various constellations. And one of the most curious thing you can observe is the Ursa Minor clock:

The constellation Ursa Minor has 7 stars shaped like a bear as the ancients saw it. They look like a clock hand to me and if you rotate the sky for 24 hours in Stellarium, you see the hand move like the hour-hand of a 24-hour clock. This constellation was also called the Phoenician Bear. With the help of Ursa Minor, the mysterious Phoenicians navigated the Mediterranean sea and worked as the bridge from the Semitic aleph-ba-ta to Greek alpha-beta and finally to the world-wide alphabet.