Keeping Time with the Skies
Introduction
This chapter explains how the Moon, Sun, and stars help us measure time. It covers why the Moon’s appearance changes, how calendars are created, and why festivals occur on different dates. It also discusses artificial satellites and their uses.
1. Why Does the Moon Look Different?
Observation: Meera saw the Moon during the day at the Kite Festival in Ahmedabad. She noticed it wasn’t a full circle and wondered why its shape changes.
Reason: The Moon reflects sunlight, not producing its own light. Its shape appears to change due to its position relative to the Earth and Sun.
Key Terms:
Phases of the Moon: The changing shapes of the Moon as seen from Earth.
Waxing (Shukla Paksha): The bright part of the Moon grows.
Waning (Krishna Paksha): The bright part of the Moon shrinks.
Full Moon (Purnima): The Moon appears as a complete bright circle.
New Moon (Amavasya): The Moon is not visible from Earth.
Why the Moon’s Shape Changes
The Moon is spherical, and half of it is always lit by the Sun.
As the Moon orbits Earth, the portion of the lit side we see changes:
Full Moon: The entire lit side faces Earth.
New Moon: The unlit side faces Earth, making it invisible.
Crescent: Less than half of the lit side is visible.
Gibbous: More than half of the lit side is visible.
The Moon takes about 29.5 days to complete one cycle of phases.
Moon’s Position in the Sky
The Moon’s position shifts daily:
- On a full Moon day, it’s opposite the Sun (rises when the Sun sets).
- During waning, it appears closer to the Sun in the sky.
- During waxing, it’s easier to spot at sunset.
Moonrise occurs about 50 minutes later each day, so the Moon can sometimes be seen during the day (e.g., in the afternoon).
2. Types of Calendars
Calendars use astronomical cycles to track time. There are three main types:
2.1 Lunar Calendars
- Based on the Moon’s phases.
- A lunar month is about 29.5 days.
- A lunar year (12 months) is 354 days, shorter than a solar year (365 days).
- Problem: Seasons shift because the lunar year is shorter.
- Example: Islamic calendar (used for Eid-ul-Fitr).
2.2 Solar Calendars
- Based on Earth’s revolution around the Sun (365.25 days).
- Months are adjusted to total 365 days (e.g., 30 or 31 days, February has 28).
- Leap Year: Every 4 years, an extra day is added (February 29) to account for the extra 0.25 days.
- To fine-tune, leap years are skipped every 100 years (e.g., 1900), but added every 400 years (e.g., 2000).
- Tropical Year: Time between two spring equinoxes (used in Gregorian calendar).
- Sidereal Year: Time for the same stars to rise at sunset again (20 minutes longer than the tropical year).
- Example: Gregorian calendar (used worldwide).
2.3 Luni-Solar Calendars
- Combine Moon phases (for months) and solar year (for seasons).
- A lunar year (354 days) is short by 11 days, so an extra month (Adhika Maasa) is added every 2-3 years.
- Examples: Hindu calendars (Amant: month starts after new Moon; Purnimant: month starts after full Moon).
- Months: Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha, Pausha, Magha, Phalguna.
2.4 Indian National Calendar
- A solar calendar starting on March 22 (day after spring equinox).
- Months have 30 or 31 days; leap years add a day to Chaitra (starts March 21).
- Adopted in 1956 based on the Surya Siddhanta, recommended by the Calendar Reform Committee (chaired by Meghnad Saha).
3. Festivals and Astronomy
Many Indian festivals are tied to the Moon’s phases (lunar or luni-solar calendars):
Diwali: New Moon in Kartika.
Holi: Full Moon in Phalguna.
Buddha Purnima: Full Moon in Vaisakha.
Eid-ul-Fitr: After sighting the crescent Moon in Ramazan.
Dussehra: Tenth day in Ashwina.
These festivals shift dates in the Gregorian calendar because lunar months are shorter.
- Luni-solar calendars adjust with an extra month, so the shift is less than a month.
Solar festivals (e.g., Makar Sankranti, Pongal, Bihu) occur on nearly the same Gregorian date, tied to the sidereal year.
- They shift slightly due to the Earth’s axis wobble (e.g., Makar Sankranti moves 1 day every 11 years).
Indian National Calendar and Festivals
- The Positional Astronomy Center publishes the Rashtriya Panchang to standardize festival dates across India, accounting for sunrise time differences.
4. Artificial Satellites
- The Moon is Earth’s natural satellite, but artificial satellites orbit Earth too.
- They appear as moving specks in the night sky, orbiting about 800 km above Earth, taking 100 minutes per orbit.
- Uses: Communication, navigation, weather monitoring, disaster management, scientific research.
ISRO’s Contributions:
Cartosat: Captures images for mapping, city planning, and disaster management (used in Bhuvan platform).
AstroSat: Observes stars and celestial objects.
Missions: Chandrayaan (Moon), Aditya L1 (Sun), Mangalyaan (Mars).
Student Satellites: AzadiSat, InspireSat-1, Jugnu.
5. Our Scientific Heritage
Ancient Indians observed the sky to create calendars, noticing the Sun’s movement:
Uttarayana: Sun moves northward (December to June).
Dakshinayana: Sun moves southward (June to December).
Texts like Surya Siddhanta and Taittiriya Samhita recorded these cycles.
Meghnad Saha: Astrophysicist who chaired the Calendar Reform Committee and developed the Saha equation for studying stars.
6. Cultural Connections
The Moon inspires Indian culture:
Music: Ragas like Chandrakalans and Shubhapantuvarali.
Dance: Mudras like Chandrakala in Bharatanatyam, Kathak, Odissi, and Kuchipudi.
Art: Moon depictions in Madhubani, Warli, Dhokra, and tribal art.
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