THE TERMINATOR SAYS I'M BACK

The Terminator is back. No, we are not talking about The Terminator movie, where Arnold Schwarzenegger portrays a tough robotic guy coming from the future with a taste for leather jackets, sunglasses and a stylish Harley. It is kind of disheartening to hear he is never coming back, as in the final movie, he said, ''This time, I won't be back'', but luckily enough, there is still another ''terminator''. Unlike the former, this one does not carry any big guns and has been visiting us twice a day for the past 4.5 billion years since the very birth of our Sun and our planet Earth and plans to visit for the next 5 billion years or so. 

This other terminator, which we speak of, is different in all aspects from the one in the movies. Earth's terminator, or in a more precise sense, for any planetary body, the terminator is a line that marks the boundary between its daytime side and the nighttime side. The terminator line, more commonly known as the twilight zone, the day-night line or the grey line, is not some physical line drawn through the surface of any planetary body but appears as a diffuse zone wherein day transitions into the night and vice versa. 

The terminator line on Earth appears as a fuzzy boundary between the day time and the night time sides
Image Credits: Public Domain, via Wikimedia Commons

The best way to have a physical understanding of the terminator line is to look up at the Moon. From a keen observation of the changing phases of the waxing and the waning Moon, we will see that the terminator line appears as a sharp contrasting boundary between the illuminated side, which faces the Sun and the darker side that faces away. However, it is worth keeping in mind that the lunar terminator is different from the Earth terminator, and why so will be explained in due course. But for now, we must take a good look at the Moon, at first, with our bare eyes or a pair of binoculars, and if possible, not a high-end telescope but a moderately good student's telescope would suffice. This will give us an idea of the importance of the terminator line. By zooming onto the terminator line, we will be able to note the finer details of the lunar topography, for the contrasting effect provided by the transition of the lunar day into the lunar night gives an additional depth by casting shadows behind elevated terrains and revealing much of the surface features including craters, ridges, and other geophysical features that are otherwise washed out under total illumination. 

The terminator line as photographed by the Lunar Reconnaissance Orbiter (LRO) appears as a sharp boundary between the day light side and the night side of the Moon.
 Image Credits: NASA

Since a planetary body is essentially spherical in shape, the terminator line appears as a complete circle whose diameter is equal to that of the planetary body. Also, owing to the rotation of the planetary body about its own axis, the terminator line does not happen to be a fixed one but moves across the surface. For tidally locked planets, that means planets that are unable to spin about their axis, their terminator line stays very nearly fixed, and in extreme scenarios of tidal locking, the line divides the planet into three distinct worlds; the scorched side that faces the parent star, the icy frosted side that faces away, and zone in between where it is neither extremely hot nor extremely cold but with comparatively moderate temperatures.  

Fortunately, Earth is not tidally locked, and as a result, the terminator line moves at the same speed as Earth's rotation, i.e., nearly equal to 1673 km/hr. From the surface, we see that the Sun rises from the East, travels overhead across the Southern part of the sky and sets in the West. However, if we could see our planet from a vantage point much higher above the North Pole (or the South Pole per se) at any particular instant of time, we would find that one half (a bit more than one half to be exact) of the Earth stays completely illuminated by the light of the Sun, while the remaining half is in total darkness, and in between, lies the transition region or the twilight zone where day gradually transitions into the night and vice versa. Any particular place passes two times through the terminator line, once at sunrise and next at sunset. This can be easily understood from the Earth's rotation. Since the planet rotates from west to east and as the terminator line forms a circle, any particular place passes through the eastern half of the terminator at dawn and the western half at dusk. The former passing results in the dawn twilight moment, while the latter forms the dusk twilight moment. 

Of all the planets in the solar system Earth has this unique twilight moments during sunset and sunrise  Image Credits: NASA Public Domain

The orientation of this terminator line depends on Earth's axial tilt of 23.5 degrees, and its rotational and orbital motions. This is easily understandable from a basic idea of how the seasons change over the course of a year, during which Earth completes one revolution around the Sun. Due to orbital and rotational dynamics, from March to September, the Northern Hemisphere inclines by 23.5 degrees towards the Sun, wherefore it experiences summer, i.e., warm temperatures, greater length of day and higher altitude of the Sun at noon, while the reverse is true for the Southern Hemisphere. Similarly, from September to March, the Southern Hemisphere is inclined by the same amount of 23.5 degrees towards the Sun, where it experiences warmer temperatures, longer days and higher altitude of the Sun at noon. During this time, the reverse happens in the Northern Hemisphere as it inclines away from the Sun and experiences winter, i.e., cool temperatures, shorter days and lower altitude of the Sun at high noon. 

Image Credits: Public Domain, via Wikimedia Commons

A more clearer picture of the orientation of the terminator line can be presented as follows. On Vernal Equinox (March 21), i.e., the day that marks the beginning of the spring (fall) season in the Northern (Southern) Hemisphere and on Autumnal Equinox (September 22), which marks the beginning of fall (spring) in the Northern (Southern) hemisphere, the Sun appears to be directly over the Equator. Therefore, on these two days, both hemispheres are equally illuminated by the Sun and consequently, they also get an equal share of day and night. Thus, the length of day and night stays the same in both hemispheres, while the North and the South poles fall precisely on the terminator line. On Summer Solstice (June 21), the North Pole is inclined to the Sun, and the latter appears to be directly overhead on the line of the Tropic Of Cancer (23.44 degrees North latitude). At this time, places on the Arctic Circle will see the Sun above the horizon during midnight and places farther North will experience daylight for 24 hours. This is what we call Midnight sun or Midsummer Nights. Now as the North Pole faces the sun, the South Pole is tilted away from the Sun. As a result, places on the Antarctic Circle see the Sun over the horizon only during midday, and all places South of it experience complete darkness. This is called Polar Night. On Winter Solstice (December 22), the Sun appears directly overhead the Tropic Of Capricorn (23.44 degrees South latitude) when the South Pole is inclined towards the Sun and the reverse of the above phenomena takes place. From the following graphic can be readily seen that on the Vernal Equinox and Autumnal Equinox, the terminator line is almost vertical whereas, on June and December Solstice the terminator line slants towards the respective Northern and Southern hemispheres. 

The terminator line shifts with the change of season
Image Credits: Public Domain, via Wikimedia Commons

We have already mentioned that the terminator line on Earth moves with a velocity of 1673 km/hour or 463 metres per second. However, this is true for places directly on and nearby the Equator. The speed of advance of the terminator line varies according to the nature of the terrain. On the other hand, the lunar terminator moves with a velocity of 15.4 kilometres per hour, i.e., about 4.27 metres per second at the Equator, which slows down and eventually becomes zero at the poles. In either case, the terminator line appears to progress faster when passing over mountainous regions as the high mountains cast a shadow over the ground in advance of the actual progress of the terminator line. Its speed slumps near the poles, where it can become almost zero and on the consecutive days of the equinoxes, it is possible to walk faster than the terminator line. Some supersonic aircraft, not to mention the International Space Station, have overcome the speed of the terminator line at the Equator and for the astronauts onboard, it is a spectacular sight to pass over the light and the dark side of the Earth. 

Image Credits: Public Domain, via Wikimedia Commons

Now that we have a sound understanding of Earth's terminator, we can finally return to the lunar terminator and elaborate on their differences. Because of the scattering of sunlight by the thick atmosphere, Earth's terminator line appears as a fuzzy zone with varying orders of illumination. Any particular place, as it passes through the termination zone, it experiences twilight, when the Sun is either rising or setting, and the sky is neither lit up nor dark but corresponds to a time which can be best described by the phrase, that long blue moment or the French expression l'heure bleue. However, on the Moon, in absence of an atmosphere, there is no scattering or refraction of light. Therefore the lunar terminator appears as a sharp ''line'' separating the illuminated side from the dark side. Also, sunrises and sunsets on the Moon appear to be quite abrupt as there is no atmosphere to scatter sunlight by low angles. Thus the Moon experiences no twilights, and as a result, the moment the Sun sets, it becomes almost completely dark as midnight, whereas on the Earth, it takes more than an hour for the night to turn into day and vice versa. Further, we know that the Moon is tidally locked and points the same face towards the Earth. Due to this tidal locking, the time it takes for the Moon to spin once about its axis is equal to the time it takes to complete one orbit around the Earth. With respect to the background stars, the Moon takes 27.3 days to complete one orbit around Earth and about 29.5 days to return to the same phase. The orientation of the terminator line changes according to the changing phases of the Moon and can be understood from the above schematic diagram. 

From satellite images gathered by the space probes we see that the terminator line on Mercury is quite similar to that of the Moon as both are tidally locked and equally devoid of an atmosphere. On the other hand, the terminator line on Venus, Earth, Mars, Jupiter, Saturn, Neptune and Uranus appears as a diffuse boundary depending on the nature of their respective atmospheres. Once again, the terminator line plays a very important role in revealing the surface features of a typical planetary body because of the contrasting effects of light and shadow. The terminator line exists for all planets, moons, large asteroids and other planetary objects in all the planetary systems in the whole universe. As a concluding remark to this article, it can be said that if we are willing to stretch our imaginations to the farthest extremes, remembering the words of Einstein, ''Imagination is more important than knowledge'', then at this very moment, on some very far away planet, farther than our technological reach, someone is looking at their setting sun.