Notes
The Moon:
Fundamental Characteristics :
- Light Source: The Moon does not produce its own light; it is visible because it reflects sunlight.
- Natural Satellite: The Moon is Earth's only natural satellite.
- Orbit: The Moon's orbit around the Earth is elliptical. A complete cycle of its phases takes approximately 29.5 days, a period defined as a lunar month.
- Rotation: The Moon takes the same amount of time to rotate on its axis as it does to revolve around the Earth. Consequently, the same side of the Moon always faces the Earth. The "New Moon" phase is a period when the side facing away from us is illuminated, making it unseen from Earth.
How Does the Moon’s Appearance Change and Why?
The changing shapes of the Moon, known as its phases, are caused by the varying angles at which we view its sunlit portion as it orbits the Earth.
It is a common misconception that the Earth's shadow causes these regular phases; the Earth's shadow is only involved during a lunar eclipse.
Phases of the Moon :
The eight phases of the Moon occur in a specific, repeating order:
- New Moon (Amavasya): The Moon is between the Earth and Sun; the sunlit side faces away from Earth, making it invisible.
- Waxing Crescent: A thin sliver of the Moon becomes visible.
- First Quarter: Half of the Moon's visible surface is illuminated.
- Waxing Gibbous: More than half of the Moon is illuminated.
- Full Moon (Purnima): The Earth is between the Sun and Moon; the entire face of the Moon visible from Earth is illuminated.
- Waning Gibbous: The illuminated portion begins to decrease.
- Last Quarter (or Third Quarter): The opposite half of the Moon is illuminated compared to the First Quarter.
- Waning Crescent: The illuminated portion shrinks back to a thin sliver before the New Moon.
Terminology:
- Waxing (Shukla Paksha): The period when the visible illuminated portion of the Moon is increasing (from New Moon to Full Moon).
- Waning (Krishna Paksha): The period when the visible illuminated portion of the Moon is decreasing (from Full Moon to New Moon).
Locating the Moon :
- Daily Rise Time: Due to its forward motion in its orbit, the Moon rises approximately 50 minutes later each day.
- Daytime Visibility: The Moon can often be seen during the day, especially during its waxing and waning phases when it is high enough in the sky while the Sun is also up. The First Quarter and Waning Gibbous phases are commonly visible during the day.
- Phase-Dependent Timings: The rise and set times of the Moon are dependent on its phase.
- New Moon: Rises and sets with the Sun, making it not visible.
- First Quarter: Is visible in the afternoon and early evening.
- Full Moon: Rises at sunset and sets at sunrise, making it visible all night.
- Last Quarter: Is visible in the late night and morning.
How Did Calendars Come into Existence? :
Humanity has historically measured time based on predictable, periodic astronomical phenomena:
- Day: Based on Earth's rotation on its axis (approximately 24 hours from one solar noon to the next).
- Month: Based on the cycle of the Moon's phases (a lunar month is ~29.5 days).
- Year: Based on Earth's revolution around the Sun (a solar year is ~365 ¼ days).
Calendar Systems :
Based on these cycles, different calendar systems have been developed:
Lunar Calendars:
- These calendars are based strictly on the lunar month.
- A year consists of 12 lunar months, totaling approximately 354 days.
- This 11-day shortfall compared to the solar year causes the seasons to drift and not align with the same months year after year.
Solar Calendars:
- These are designed to synchronize with the solar year and the seasons.
- The widely used Gregorian calendar is a solar calendar.
- It uses months of varying lengths (28, 30, or 31 days) and incorporates a leap year (adding an extra day every four years) to account for the Earth's revolution taking approximately 365.25 days.
Luni-solar Calendars:
- This hybrid system primarily uses the Moon's phases for months but makes adjustments to stay aligned with the seasons.
- To correct for the discrepancy between the lunar and solar years, an extra intercalary month (known as Adhika Maasa in India) is added to the calendar every two to three years.
Solar Year Variants :
- Tropical Year: The time between two successive spring equinoxes. This is the basis for the Gregorian calendar, as it aligns directly with the cycle of seasons.
- Sidereal Year: The time required for the Earth to complete one orbit around the Sun relative to the fixed stars. It is about 20 minutes longer than the tropical year and is used by astronomers to track the Earth's position in its orbit.
The Indian National Calendar :
India has a rich heritage of astronomical observation and utilizes diverse calendar systems that are deeply integrated with cultural and religious practices.
For official purposes, the Government of India uses a standardized solar calendar alongside the Gregorian calendar.
- Structure: It is a solar calendar with 365 days in a regular year. The year begins on March 22 (the day after the spring equinox), or March 21 in a leap year.
- Months: The month names are taken from traditional Indian calendars (Chaitra, Vaisakha, Jyeshtha, etc.). The second through sixth months have 31 days, and the rest have 30.
- Leap Years: Leap years are synchronized with the Gregorian calendar, with the extra day added to the first month, Chaitra.
- Origin: The calendar was adopted in 1956 following recommendations from the Calendar Reform Committee, chaired by astrophysicist Meghnad Saha.
Traditional and Luni-solar Systems
Many traditional calendars used across India are luni-solar.
- Month Start/End: They can be categorized as Amant (the new month begins the day after the new Moon) or Purnimant (the new month begins the day after the full Moon).
- Ancient Observations: Ancient Indian texts like the Taittirīya Saṁhitā and Surya Siddhanta record detailed observations of the Sun's apparent annual north-south movement (Uttarayan and Dakshinayan), which are linked to the seasons.
Are Festivals Related to Astronomical Phenomena?
The dates of many Indian festivals are determined by astronomical events.
- Lunar-Based Festivals: Festivals like Diwali (new Moon of Kartika), Holi (full Moon of Phalguna), and Eid-ul-Fitr (sighting of the crescent Moon) are tied to specific lunar phases. Consequently, their dates on the Gregorian calendar shift each year.
- Calendar Impact: For festivals based on purely lunar calendars like Eid-ul-Fitr, the date moves about 11 days earlier in the Gregorian calendar each year, allowing the festival to occur in any season over time. For festivals based on luni-solar calendars, the intercalary month keeps the date within a roughly one-month window relative to the seasons.
- Solar-Based Festivals: Some festivals, such as Makar Sankranti, Pongal, and Vaisakhi, follow a solar sidereal calendar and thus occur on nearly the same date in the Gregorian calendar each year.
Eclipses and Gravitational Effects :
The Nature of Eclipses
Eclipses are distinct from lunar phases and occur only when the Sun, Earth, and Moon are perfectly aligned, allowing one body to cast a shadow on another. Because the Moon's orbit is tilted relative to Earth's orbit, this perfect alignment does not happen every New Moon and Full Moon.
Comparison of Solar and Lunar Eclipses
| Feature | Solar Eclipse | Lunar Eclipse |
| Alignment | The Moon comes between the Sun and Earth. | The Earth comes between the Sun and Moon. |
| Moon Phase | New Moon | Full Moon |
| Mechanism | The Moon's shadow falls on the Earth. | The Earth's shadow falls on the Moon. |
| Visibility | Visible only from a limited, specific area on Earth. | Visible from anywhere on the night side of Earth. |
| Frequency | Less frequent to observe from a single given location. | More frequent to observe and longer lasting. |
| Safety | It is dangerous to look directly at a solar eclipse without special filters or glasses, as it can cause permanent eye damage. | Safe to view with the naked eye. |
Tides :
The Moon's gravitational pull is the primary cause of tides on Earth. Tides are at their highest during the New Moon and Full Moon phases, when the gravitational forces of the Sun and Moon are aligned.
Why Do We Launch Artificial Satellites in Space?
Artificial satellites are man-made objects placed in orbit around the Earth for a variety of purposes, including:
- Communication
- Navigation
- Weather monitoring and forecasting
- Disaster management
- Scientific research
- Earth observation and mapping
India's Space Program (ISRO)
Pioneered by Dr. Vikram Sarabhai, known as the Father of the Indian Space Programme, the Indian Space Research Organisation (ISRO) has launched numerous satellites and missions.
Key Satellites:
- Cartosat Series: Capture high-resolution images of Earth for mapping, urban planning, and disaster management, supporting platforms like Bhuvan.
- AstroSat: An astronomical observatory mission to study celestial objects.
Major Missions:
- Chandrayaan (1, 2, 3): Missions to the Moon.
- Mangalyaan: Mission to Mars.
- Aditya L1: Mission to study the Sun.
Challenges :
- The increasing number of satellites has led to the problem of space debris or space junk.
- Defunct satellites and rocket parts crowd orbits and pose a collision risk to operational satellites.
International efforts are underway to address this growing hazard.
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