Tilted axis of the Earth and its motion around the Sun:
The Earth rotates on an imaginary line called its axis, which is tilted about \(23.5^{\circ}\) from the upright position with respect to its orbit around the Sun. This tilt remains constant as the Earth revolves around the Sun. Because of the Earth’s spherical shape and tilted axis, the Sun’s rays do not fall equally on all parts of the Earth throughout the year. As a result, different regions receive different angles and durations of sunlight, which leads to seasonal changes.
The Earth’s axis is tilted about \(23.5^{\circ}\) from the upright position relative to its orbit around the Sun, and this tilt remains constant as the Earth revolves. Because of this, the Sun’s rays do not fall equally on all parts of the Earth throughout the year, leading to seasonal changes.
Summer conditions in the Northern Hemisphere (June Position of the Earth):
In June, the Northern Hemisphere is tilted towards the Sun. As a result, the Sun’s rays fall more directly on this hemisphere and are concentrated over a smaller area, making the sunlight more intense. At the same time, the Southern Hemisphere is tilted away from the Sun, so the same amount of sunlight is spread over a larger area, making it less intense.
During this period, places in the Northern Hemisphere receive more than \(12\ hours\) of daylight, while places in the Southern Hemisphere experience shorter days. The North Pole receives sunlight for all \(24\ hours\), leading to continuous daylight called polar day. The combined effect of higher intensity sunlight and longer daylight hours causes the summer season in the Northern Hemisphere.
Winter conditions in the Northern Hemisphere (December position of the Earth):
In December, the situation is exactly the opposite. The Southern Hemisphere is tilted towards the Sun and receives more direct and intense sunlight that are concentrated over a small area, while the Northern Hemisphere is tilted away from the Sun and receives slanting rays that are spread over a larger area and are therefore less intense.
Due to the Earth’s rotation, places in the Southern Hemisphere enjoy longer daylight hours, whereas the Northern Hemisphere has shorter days. During this time, the North Pole remains in complete darkness for \(24\ hours\), experiencing what is known as the polar night.
These reduced daylight hours and weaker sunlight result in the winter season in the Northern Hemisphere. These variations in sunlight intensity and day length clearly explain how the Earth’s tilted axis and rotation cause seasons.
Common misconceptions about the causes of seasons:
Two incorrect explanations are often given to explain why seasons occur on the Earth.
1. One mistaken idea is that when the Northern Hemisphere tilts towards the Sun, the Earth is closer to the Sun.
2. Another incorrect belief is that the Earth’s orbit is highly oval, with the Sun away from the centre, causing large changes in the Earth–Sun distance during the year.
In reality, the difference in the Earth–Sun distance in both these cases is very small and does not cause seasons. In fact, the Earth is closest to the Sun in January, when the Northern Hemisphere experiences winter. Seasons actually occur due to the tilt of the Earth’s axis and not because of changes in the distance from the Sun.
Solstices and equinoxes in the Northern Hemisphere:
In the Northern Hemisphere, the longest day and the shortest night occur around \(21\ June\), a day known as the summer solstice.
After the summer solstice, the length of daylight gradually decreases, while the duration of night increases. The shortest day and the longest night occur around \( 22\ December\), which is called the winter solstice.
On about \( 21\ March\) and \( 23\ September\), day and night are nearly equal, each lasting about \(12\ hours\). These two days are known as the spring equinox and the autumn equinox, respectively, in the Northern Hemisphere.
Day and night at the Polar regions:
- At the North Pole, the Sun rises on \(21\ March\) and remains continuously above the horizon for about six months, resulting in uninterrupted daylight. The Sun sets around \(22\ September\), after which the region experiences six months of continuous darkness.
- In contrast, at the South Pole, the pattern is exactly reversed. The Sun rises around \(22\ September\) a nd stays visible for the next six months, while it sets around \(21\ March\) , leading to a long period of darkness.
Thus, both polar regions experience six months of day and six months of night, but these periods occur at opposite times of the year. This phenomenon is very different from most other parts of the Earth, where day and night alternate every \(24\ hours\) due to the Earth’s rotation.
Seasonal conditions near the Equator:
At the equator, the duration of day and night remains nearly equal, with about \(12\ hours\) of sunlight and \(12\ hours\) of darkness throughout the year. The intensity of the Sun’s rays falling on the equatorial region does not change much in different months. Because of this, the effect of seasons is less noticeable in regions close to the equator, such as the southern states of India.
In addition to latitude, other factors like local geographical features and proximity to oceans or seas also influence climate patterns. These factors can modify the general seasonal patterns observed in the Northern and Southern Hemispheres.
Solar eclipse:
At certain times, the Moon moves between the Sun and the Earth in such a position that it blocks the sunlight from reaching the Earth. This phenomenon is called a solar eclipse.
Transit of Venus and apparent size of planets:
Although the planets Mercury and Venus are much larger than the Moon, they are located much farther away from the Earth. Due to this greater distance, their apparent sizes are much smaller than that of the Sun, so they are unable to block sunlight completely. For instance, when Venus moves between the Sun and the Earth, it appears as a small black dot crossing the bright surface of the Sun. This rare phenomenon is known as the Transit of Venus.
Total Solar eclipse:
A total solar eclipse is a phenomenon in which the Moon comes between the Sun and the Earth in such a way that the Moon’s shadow falls on a small area of the Earth, causing complete darkness in that region and making the Sun completely invisible to observers there.
Total solar eclipse
Partial Solar eclipse:
A partial solar eclipse occurs when the Moon comes between the Sun and the Earth and blocks only a part of the Sun. As a result, some sunlight still reaches the Earth, and observers can see only a portion of the Sun covered by the Moon.
Safe practices to observe a solar eclipse:
Use solar eclipse glasses or certified solar filters to view the Sun.
Observe the Sun indirectly, for example, by projecting its image using a pinhole projector or a mirror setup.
Attend eclipse-viewing programmes organised by astronomy clubs or planetariums, where proper precautions are provided.
Lunar eclipse:
A lunar eclipse is an event that occurs when the Earth comes between the Sun and the Moon, blocking sunlight from reaching the Moon.
During this event, we can observe the Earth’s shadow falling on the Moon.
Lunar eclipse
Total lunar eclipse:
A total lunar eclipse occurs when the Earth passes directly between the Sun and the Moon, causing the Earth to cast its full shadow (the umbra) across the entire lunar surface.
During this alignement, the Moon often appears reddish, know as a "Blood Moon", due to sunlight refracting through the Earth's atmosphere.
Partial lunar eclipse:
A partial lunar eclipse occurs when only part of the Moon passes into the Earth’s shadow, so only a portion of the Moon is covered, while the rest remains illuminated.
Kodaikanal Solar Observatory:
The Kodaikanal Solar Observatory is situated in the scenic Palani hills of southern India. Established in \(1899\), it has been collecting valuable information about the Sun for more than a century. The observatory is managed by the Indian Institute of Astrophysics (IIA), Bengaluru.
M.K. Vainu Bappu: Father of Modern Indian Astronomy
M.K. Vainu Bappu is regarded as the father of modern Indian astronomy. He played a key role in establishing several telescopes and instruments across India, including those at Manora Peak near Nainital (Uttarakhand) and Kavalur (Tamil Nadu). The Kavalur observatory has been named in his honor. Bappu primarily studied stars and even discovered a comet. He also traveled to various parts of the world to observe solar eclipses.