Astronomy News
How the Moon Influences the Bangkok’s Flood
Since September 24, 2026, heavy rainfall has stormed central Thailand causing massive flooding as the city struggles to manage and drain the water down Chao Phraya river. However, there is one unlikely influential body that complicates these drainage efforts and even at times halts it completely twice a day: The Moon.
But how can the Moon, located 400,000 km away, have any effect on the water levels of Bangkok?
The Moon, like everything with mass in the universe, exerts a gravitational pull on everything around it. The strength of this force relies not only on the masses of the objects, but also on the distance between them. This is known as Newton’s Law of Universal Gravitation. When the Moon is above your head, you are being pulled towards it. However, the ground beneath you is also pulled towards the Moon by a similar amount, which is still weaker than the Earth’s gravitational pull keeping you on the ground. This is why you do not need to worry about being yanked towards the Moon anytime soon.
Since your head is slightly closer towards the Moon, it experiences a stronger gravitational pull than the ground does. However the difference is incredibly small, amounting to only about one ten-thousandth of one billionth of Earth's normal gravity, so we do not normally feel its effect. However, for the Earth’s ocean, where some part of the water is experiencing the Moon's gravity 6,000 km closer, this effect becomes much more significant. The Moon’s gravity pulls more strongly on the water closer to it and less strongly on the water farther away. This results in the phenomenon known as “tide”, and the force of gravity is called the “tidal force”.
The Moon is not the only thing that affects Earth’s tides, the Sun also exerts tidal force. Although it is more massive than the Moon, the tidal influence from it is only a third that of the Moon due to its greater distance from Earth. These two tidal forces interplay in a rather complex pattern, but it lines up twice a month during the full and new moon, causing strong tides known as “spring tide” then partially cancel each other during the first and third quarter, causing a weaker tide known as “neap tide”.
So, what does this have to do with Bangkok?
Bangkok is located on the lower Chao Phraya River, with an average elevation of only 1.5m above sea level. This means that during high tide, the river drains much less effectively. In fact, Chao Phraya River even flows backwards sometimes during the highest tides.
Even though spring tide also comes with extremely low tide, the extra elevation difference barely adds additional drainage effects and as a result barely compensate for the blockage and potential backflow caused by high tide.
Although heavy rainfall plays a major role in the recent flooding, the Full Moon on the 26th of September resulted in stronger tidal effects that may have added to the challenge as authorities work to drain floodwaters from the Chao Phraya River basin. It offers another glimpse into how celestial bodies can have a direct influence on the water beneath us.
Text: Rika Mei Reventar, Matipon Tangmatitham