ដំណាក់កាល​ព្រះច័ន្ទ និង​ការ​លិច​ព្រះអាទិត្យ

The Moon never changes shape. Half of it is always lit by the Sun;what changes is how much of that lit half faces you. Once you see why, eclipses stop being mysterious and start being predictable.

Half of the Moon is always in sunlight. The phases are just our point of view.

មើល

The Moon does not make its own light. It shines because sunlight falls on it. At any moment, one half of the Moon is lit by the Sun and the other half is dark, just like Earth has day and night.

As the Moon goes around Earth, we see different amounts of its sunlit half:

• New Moon: the Moon is between Earth and the Sun. The lit half faces away from us, so we cannot see it.
• First quarter: we see half of the disc lit.
• Full Moon: Earth is between the Sun and the Moon. We see the whole lit half.
• ត្រីមាសចុងក្រោយ: ពាក់កណ្តាលភ្លើងម្តងទៀត, ខាងក្រៅផ្សេងទៀត.

In the tool, the left picture looks down on Earth and the Moon from far above, with the Sun off to the left. The right picture shows what you see from Earth. Move the slider and watch both change together.

Sometimes the Sun, Earth and Moon line up exactly:

• A solar eclipse happens when the Moon’s shadow falls on Earth at new Moon.
• A lunar eclipse happens when Earth’s shadow falls on the Moon at full Moon.

យល់

The Moon goes once around Earth, measured against the stars, in 27.3 days (the sidereal month). ប៉ុន្តែដំណាក់កាលធ្វើឡើងវិញរាល់ 29.5 ថ្ងៃ (ខែ synodic) ។ ហេតុអ្វី​បានជា​ខុសគ្នា? While the Moon goes around Earth, Earth moves about 1/12 of the way around the Sun. The Moon needs about two more days to catch up with the new Sun direction.

ភាគ​នៃ​ដ្រាយ​ដែល​មើល​ទៅ​ដូច​ជា​បាន​បើក​ឡើង​អាស្រ័យ​លើ​មុំ​រវាង​ព្រះអាទិត្យ និង​ព្រះច័ន្ទ​ដូច​ដែល​ឃើញ​ពី​ផែនដី ដែល​ហៅ​ថា រយៈទទឹង:

ការ​ពង្រីក	ដំណាក់កាល	អាន​ប្រភាគ
0°	ផ្កាយ​ថ្មី	0%
90°	ត្រីមាស​ទីមួយ	50%
180°	ព្រះច័ន្ទ​ពេញវង់	100%
270°	ត្រីមាស​ចុងក្រោយ	50%

If the Moon went around exactly in the same plane as Earth goes around the Sun, we would get a solar eclipse every new Moon and a lunar eclipse every full Moon. But the Moon’s orbit is tilted by about 5.1 degrees. Usually the Moon passes a little above or below the Sun-Earth line, and nothing happens.

ចំណុច​ពីរ​ដែល​អ័ក្ស​ព្រះច័ន្ទ​ឆ្លងកាត់​ផ្ទៃ​អ័ក្ស​របស់​ផែនដី​ត្រូវ​បាន​ហៅ​ថា ថ្នាំង. Eclipses can only happen when a new or full Moon falls close to a node. This happens in two “eclipse seasons” each year, about six months apart.

មេ

The illuminated fraction k follows from the phase angle i (the Sun-Moon-Earth angle):

k = (1 + cos i) / 2

ដោយសារ​ព្រះអាទិត្យ​មាន​ចម្ងាយ 390 ដង​ពី​ភព​ព្រះច័ន្ទ i ≈ 180° − elongation ។ ឧបករណ៍​គណនា​ខ្ញុំ​ជាមួយ​ស៊េរី Meeus (ក្បួនដោះស្រាយ​តារាសាស្ត្រ, eq. 48.4), which corrects the mean elongation for the main periodic terms of the Moon’s and the Sun’s orbits. It places new and full Moons to within about half an hour of the precise times, plenty for learning.

ខែ​ស៊ីណូដិក​គឺ​ជា​ការ​អនុវត្ត​តាម​ល្បឿន​មុំ​ពីរ​: 1/P_syn = 1/P_sid − 1/P_year = 1/27.3217 − 1/365.256, ផ្តល់​ឲ្យ​មាន​រយៈពេល 29.531 ថ្ងៃ។

Whether an eclipse is possible depends on the Moon’s argument of latitude F, its angle from the ascending node. Meeus ផ្តល់នូវច្បាប់ស្កេនរហ័សមួយ៖ នៅ​ពេល​ព្រះច័ន្ទ​ថ្មី ឬ​ពេញលេញ បើ |sin F| > 0.36 (F ច្រើនជាង 21° ពី​ចំណុច) គ្មាន​ការ​រះ​ទេ ។ The tool applies exactly this test. It is a necessary condition, not a prediction of the eclipse type or where it is visible: that needs the Moon’s distance, Earth’s shadow geometry and much more precise positions.

The nodes are not fixed. ពួកគេរត់ត្រឡប់មកវិញជុំវិញ ecliptic ម្តងក្នុងមួយឆ្នាំ 18.6 ដែលជាមូលហេតុដែលរដូវផ្ការីកមកដល់ប្រហែល 19 ថ្ងៃមុនរៀងរាល់ឆ្នាំនិងផ្នែកនៃមូលហេតុដែលផ្ការីកកើតឡើងឡើងវិញនៅក្នុងវដ្តល្បី 18 ឆ្នាំ saros ។