Chapter 11
Keeping Time with the Skies
π€ Probe and Ponder
Have you ever seen the Moon during the day, and wondered why? If we didn't have clocks or calendars, how else could we measure time? What would happen to the night sky if Earth had two moons instead of one?
At a kite festival in Ahmedabad during Makar Sankranti, Meera spotted the Moon shining in the daytime sky β not as a full circle, since she knew its shape changed every night, but it got her wondering: the Moon is spherical and shines only by reflecting sunlight, so why isn't the whole Moon visible every night?
How Does the Moon's Appearance Change and Why?
Watching the Moon at sunrise or sunset over about a month reveals a clear pattern: its bright portion grows or shrinks a little each day, and it appears to shift position in the sky relative to the Sun from one day to the next.
Phases of the Moon
The bright portion of the Moon shrinks from a full circle to a half circle in about a week, then keeps shrinking for another week until it's no longer visible β this two-week shrinking stretch is the waning period. The day the Moon appears as a full bright circle is full Moon day (Purnima); the day it isn't visible at all is new Moon day (Amavasya). After new Moon, the bright side grows back to a half circle in a week and to a full circle in another week β this growing stretch is the waxing period. In India, the waning period is called Krishna Paksha and the waxing period Shukla Paksha, cycling one after the other, with a full cycle from one full Moon to the next taking about a month. These changing shapes of the Moon's bright portion, seen from Earth day to day, are called the phases of the Moon.
The Moon's phase also decides where and when to spot it: on full Moon day it's nearly opposite the Sun, rising as the Sun sets; as its bright part shrinks toward a half circle, the Moon appears overhead at sunrise; a few days later, as a crescent, it's even closer to the Sun in the sky. A waxing Moon is easiest to spot at sunset, and a waning Moon at sunrise β which is also why the Moon always rises and sets at different times than the Sun.
β¬ A Step Further
Many people assume the Moon rises when the Sun sets, but that's not always true β the Moon actually rises about 50 minutes later each day. Sometimes moonrise happens in the afternoon, so the Moon can be spotted in the eastern sky in broad daylight (it just needs about 30 minutes past the listed moonrise time to climb high enough to see).
The Moon doesn't emit its own light β it shines by reflecting sunlight, so only the half of it facing the Sun is illuminated at any moment. As the Moon orbits Earth, only one of its halves ever faces us, but that Earth-facing half isn't always the illuminated one β sometimes we see all of the illuminated portion, sometimes only part of it, and on new Moon day, none of it at all, since the non-illuminated half faces Earth then. That's why the Moon looks different on different days.
Holding a ball on a stick at arm's length under a lamp, and slowly turning around while watching it, recreates this exactly: held opposite the lamp, the whole lit side faces you (like full Moon); held toward the lamp, only the dark side faces you (like new Moon); at every position in between, a curved line separates the lit and dark portions, just like the Moon's changing shapes. Mapping this onto the Moon's real orbit: at positions where more than half the illuminated side is visible, that's the gibbous phase; where less than half is visible, that's the crescent phase β and since the Moon takes about a month to orbit while Earth rotates once a day, people across the whole planet see nearly the same phase on any given day.
β¬ A Step Further
The Moon's phases are NOT caused by Earth's shadow falling on it β that's a common misconception. Phases happen because of the changing relative orientation of the Sun, Moon, and Earth as the Moon orbits. Earth's shadow on the Moon instead causes a lunar eclipse, which (like a solar eclipse, on new Moon day) can only happen on full Moon day β and even then, not every month, because the Moon's orbit is slightly tilted relative to Earth's orbit around the Sun.
How Did Calendars Come into Existence?
The Sun's apparent daily rise in the east and set in the west β really caused by Earth's rotation on its axis β is the foundation of the day. The average time from the Sun's highest point in the sky on one day to its highest point the next is 24 hours, called the mean solar day; that highest point can be found by tracking when an object's shadow is shortest during the day.
The Moon's phases give a longer natural cycle β about 29.5 days β forming the basis for a month. Earth's revolution around the Sun, taking nearly 365 and a quarter days and carrying Earth through one full cycle of seasons, defines a solar year.
Lunar Calendars
Ancient observers noticed that roughly 12 cycles of the Moon's phases fit into one cycle of seasons β giving rise to lunar calendars, built from a day, a month of about 29.5 days, and a lunar year of 12 lunar months (354 days). Because the seasons actually repeat over about 365 days, a purely lunar year drifts out of sync with the seasons year after year.
Solar Calendars
Since knowing when seasons would arrive mattered enormously for agriculture, solar calendars were created to stay synchronised with the seasons instead β the Gregorian calendar, in wide use today, is one. Its months are adjusted to add up to 365 days (hence months of 30 or 31 days, and February's 28), and since Earth actually takes nearly an extra quarter-day per year, an extra day is added every four years as a leap year to stay synchronised β in the Gregorian calendar, a year divisible by four gets a 29-day February.
β¬ A Step Further
Adding a leap day every four years actually overcorrects very slightly over time, since Earth takes a touch less than 365.25 days between successive spring equinoxes. To fix this, century years like 1700, 1800, and 1900 skip the leap year β but skipping every century year would undercorrect instead, so every 400 years (like 1600 and 2000) the leap year is added back after all. These careful adjustments keep the calendar closely matched to the seasons over long stretches of time.
β¬ A Step Further
The time between successive spring equinoxes is called the tropical year, which the Gregorian calendar is based on. The time for the same stars to rise again at sunset is the sidereal year β longer than the tropical year by only about 20 minutes, so the two calendars stay nearly identical for a long time. Modern astronomers use the sidereal year to track Earth's position in its orbit around the Sun.
π Our Scientific Heritage
Long before knowing Earth revolves around the Sun, and without modern instruments, ancient observers β including in India β tracked the sky carefully enough to determine the year was about 365 days. They noticed the Sun doesn't always rise exactly in the East: a little northward in summer, a little southward in winter, reaching these extremes at the solstices around June 21 and December 21. The Sun's apparent northward drift from December to June is called Uttarayan, and its southward drift from June to December, Dakshinayan β the TaittirΔ«ya SaαΉhitΔ (6.5.3) records: "Thus the Sun moves southwards for six months and northwards for six months." Equinoxes and solstices were also tracked by watching which stars rose at sunset; the Surya Siddhanta notes that the constellation Capricorn (Makar) sat behind the Sun around the winter solstice in ancient times.
Luni-Solar Calendars
Twelve lunar months add up to 354 days, falling about 11 days short of the solar year β so every 2β3 years the shortfall adds up to nearly a full month, corrected by inserting an extra intercalary month (Adhika Maasa) to realign the calendar. Calendars that do this are called luni-solar calendars, blending elements of both lunar and solar calendars, and widely used across India.
π‘ Ever Heard Of...
Indian luni-solar calendar months carry names like Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha, Pausha, Magha, and Phalguna. In Amant calendars, a month starts the day after new Moon and ends on new Moon day; in Purnimant calendars, a month starts the day after full Moon and ends on full Moon day.
The Indian National Calendar
India's national calendar, used alongside the Gregorian calendar for official purposes, is a solar calendar of 365 days beginning on 22 March (the day after the spring equinox), with months of 30 or 31 days named after traditional Indian calendars. Its leap years are matched to the Gregorian calendar by adding a day to Chaitra, the first month.
π‘ Ever Heard Of...
In 1952, the Government of India set up a Calendar Reform Committee to recommend one accurate, uniform calendar for the whole country. Its recommended "Unified National Calendar" took effect from 21 March 1956 CE (1 Chaitra 1878 Saka), following the same general principles as the ancient Surya Siddhanta.
π¬ Be a Scientist
Meghnad Saha (1893β1956) was a pioneering Indian astrophysicist who studied stars and their temperatures, developing the famous Saha equation. The Saha Institute of Nuclear Physics in Kolkata is named after him, and he chaired the Calendar Reform Committee.
Are Festivals Related to Astronomical Phenomena?
Many Indian festivals follow the Moon's phases, tied to lunar or luni-solar calendars β Diwali on the new Moon of Kartika, Holi on the full Moon of Phalguna, Buddha Purnima on the full Moon of Vaisakha, Eid-ul-Fitr at the sighting of the crescent Moon ending Ramazan, and Dussehra on the tenth day of Ashwina β so each falls on a different Gregorian date most years. Luni-solar festival dates shift too, but by less than a month, since the intercalary month periodically corrects the lunar-solar gap; purely lunar festivals like Eid-ul-Fitr have no such correction, so they can drift through every month of the Gregorian year over time.
β¬ A Step Further
Festivals like Makar Sankranti, Pongal, Bihu, Vaisakhi, Poila Baisakh, and Puthandu follow a solar sidereal calendar instead, landing on almost the same Gregorian date every year. Long ago these festivals were tied to a solstice or equinox, but the tiny gap between the sidereal and tropical years β caused by Earth's axis slowly wobbling, like a wobbling top β makes their dates drift gradually away from those solstices/equinoxes: Makar Sankranti, for instance, shifts about one day every 71 years.
π‘ Ever Heard Of...
Since many festival dates depend on the exact lunar phase at sunrise, and sunrise happens earlier in Eastern India than Western India, the same festival can fall a day apart between regions in the same year. To keep dates uniform nationwide, the Positional Astronomy Centre publishes the Rashtriya Panchang every year β detailed Sun and Moon position calculations for a central Indian location β advising the Government of India on festival dates for holidays.
π‘ Ever Heard Of...
The Moon has long inspired Indian art: ragas like Chandrakauns, Chandranandan, and Shubhapantuvarali evoke it in classical music, while mudras like Chandrakala and Ardhachandran carry its imagery into Bharatanatyam and other dance forms like Kathak, Odissi, and Kuchipudi. Traditional painting styles such as Madhubani and Warli, along with sculpture and pottery from communities like the Saura and Gond, prominently depict the Moon and Sun too.
Why Do We Launch Artificial Satellites in Space?
Besides the Moon β Earth's natural satellite β many artificial satellites launched by countries around the world orbit Earth too, appearing as tiny moving specks in the night sky. Most orbit around 800 km up, completing one orbit roughly every 100 minutes, and support communication, navigation, weather monitoring, disaster management, and scientific research β the Indian Space Research Organisation (ISRO) has launched many satellites for exactly these purposes. Spotting one just needs a clear sky, just before sunrise or after sunset: look for a steady or flickering point of light moving quickly and steadily across the sky, unlike a plane's blinking lights or a star's stillness.
β¬ A Step Further
As more countries send satellites into space, many retired satellites and rocket parts become space junk, crowding orbits and threatening collisions with working satellites. Small debris burns up re-entering the atmosphere, but larger pieces can reach the ground β countries are now working together to clean this debris up.
π Our Scientific Heritage
ISRO's Cartosat satellites capture high-quality Earth images used for mapping, city planning, and disaster response β feeding the Bhuvan platform's terrain, soil, and land-use data. AstroSat makes scientific observations of stars and celestial objects. India's other missions include Chandrayaan 1, 2, and 3 to the Moon, Aditya L1 to study the Sun, and Mangalyaan to Mars β and ISRO even lets students build and launch small satellites like AzaadiSat, InspireSat-1, and Jugnu.
π¬ Be a Scientist
Vikram Ambalal Sarabhai (1919β1971), known as the Father of the Indian Space Programme, pioneered India's first artificial satellite launches. The Vikram Sarabhai Space Centre in Thiruvananthapuram, ISRO's rocket and launch-vehicle development centre, is named after him.
β¬ A Step Further
Near a coastline, water levels rise and fall in a regular pattern called tides β much like moonrise, a given tide returns about 50 minutes later each following day. Careful observation shows tide levels are closely tied to the Moon's position and phase.
π Snapshots
- The illuminated part of the Moon changes shape from day to day through phases, like the new Moon, crescent, and full Moon.
- The phases of the Moon happen because we see different parts of the Moon's illuminated portion as it moves around the Earth.
- A full cycle of the Moon's phases takes about a month.
- The various cycles observed in nature led to the creation of calendars.
- Lunar calendars follow the Moon's cycle, solar calendars follow the cycle of seasons (tied to Earth's position around the Sun), and luni-solar calendars adapt to both.
- Artificial satellites are human-made, launched from Earth, and provide important information for our well-being and space-science studies.
π― Keep the Curiosity Alive
- State whether each statement is True or False: (i) We can only see the part of the Moon that reflects sunlight toward us. (ii) Earth's shadow blocks sunlight from reaching the Moon, causing its phases. (iii) Calendars are based on various astronomical cycles that repeat predictably. (iv) The Moon can only be seen at night.
- Amol was born on 6th May on a full Moon day. Does his birthday fall on a full Moon day every year? Explain your answer.
- Given a diagram showing the Sun, Earth, and Moon with the illuminated Moon portion marked, name two things that are shown incorrectly.
- Given six pictures of the Moon in different phases: (i) match each picture to its phase β three days after New Moon, Full Moon, three days after Full Moon, a week after Full Moon, and the day of New Moon; (ii) list which of the pictured phases are never actually seen from Earth (use your observations from tracking the Moon, or the waxing/waning cycle, as a reference).
- Malini saw the Moon overhead in the sky at sunset. (i) What phase of the Moon did she see? (ii) Is the Moon in its waxing or waning phase?
- Ravi said, "I saw a crescent Moon rising in the East when the Sun was setting." Kaushalya said, "Once I saw a gibbous Moon during the afternoon in the East." Who is telling the truth?
- Scientific studies show the Moon is slowly moving farther from Earth and slowing in its revolution. Will luni-solar calendars need an intercalary month more often, or less often?
- A total of 37 full Moons happen during 3 years in a solar calendar. Show that at least two of the 37 full moons must fall in the same month of the solar calendar.
- On a particular night, Vaishali saw the Moon in the sky continuously from sunset to sunrise. What phase of the Moon would she have seen?
- If we stopped having leap years, in approximately how many years would Indian Independence Day shift into winter?
- What is the purpose of launching artificial satellites?
- Which periodic phenomenon is each of these time units based on: (i) day (ii) month (iii) year?