In science
The atomic number of zirconium.
Astronomy
The planet Venus forms a pentagram in the night sky every eight years with it returning to its original point every 40 years with a 40-day regression (some scholars believe that this ancient information was the basis for the number 40 becoming sacred to Jews, Christians, and Muslims).
In religion
The number 40 is used in Jewish, Christian, Islamic, and other Middle Eastern traditions to represent a large, approximate number, similar to "umpteen".
Judaism
In the Hebrew Bible, forty is often used for time periods, forty days or forty years, which separate "two distinct epochs".[1]
Rain fell for "forty days and forty nights" during the Flood. (Genesis 7:4)
Spies explored the land of Israel for "forty days." (Numbers 13:25)
The Hebrew people lived in the Sinai desert for "forty years". This period of years represents the time it takes for a new generation to arise. (Numbers 32:13)
Moses' life is divided into three 40-year segments, separated by his growing to adulthood, fleeing from Egypt, and his return to lead his people out. (Acts 7:23,30,36)
Several Jewish leaders and kings are said to have ruled for "forty years", that is, a generation. Examples include Eli (1 Samuel 4:18), Saul (Acts 13:21), David (2 Samuel 5:4), and Solomon (1 Kings 11:42).
Goliath challenged the Israelites twice a day for forty days before David defeated him. (1 Samuel 17:16)
Moses spent three consecutive periods of "forty days and forty nights" on Mount Sinai.
40 lashes is one of the punishments meted out by the Sanhedrin, though in actual practice only 39 lashes were administered.
One of the prerequisites for a man to study Kabbalah is that he is forty years old.
Christianity
Christianity similarly uses forty to designate important time periods.[1]
Before his temptation, Jesus fasted "forty days and forty nights" in the Judean desert. (Matthew 4:2, Mark 1:13, Luke 4:2)
Forty days was the period from the resurrection of Jesus to the ascension of Jesus. (Acts 1:3)
Biblical verse Numbers 14:33-34 alludes to the same with ties to the prophecy in The Book of Daniel. "For forty years--one year for each of the forty days you explored the land--you will suffer for your sins and know what it is like to have me against you.'"
In modern Christian practice, Lent consists of the 40 days preceding Easter. In much of Western Christianity, Sundays are excluded from the count; in Eastern Christianity, Sundays are included.
The Forty Martyrs of Sebaste
Kirk Kilisse, "Forty Churches" (Σαράντα Εκκλησιές) in Eastern Thrace
Islam
Muhammad was forty years old when he first received the revelation delivered by the archangel Gabriel.
Masih ad-Dajjal roams around the Earth in forty days, a period of time that can be as many as forty months, forty years, and so on.
God forbade the Fasiqun (the non-believers) from entering the Holy Land for 40 years to separate them from Musa (Moses) and his brother.[3]
Musa (Moses) spent 40 days on Mount Sinai where he received the 10 commandments.[4]
Prophet Ibrahim spent 40 days in a fire and lived because God made the fire like flowers.[citation needed]
On the 40th verse (ayat) of the 2nd chapter of the Quran (Al-Baqarah) God changes the topic.
Forty was the number of days that Prophet Ilyas (Elijah) spent in the wilderness before the angel appeared to him with God's message on Mount Horeb.[citation needed]
Forty was the number of days that Prophet Isa (Jesus) was tempted in the desert by Satan.[citation needed]
Muhammad praying and fasting in the cave for 40 days.[citation needed]
Muhammad then had 40 followers to spread the religion of Islam.[citation needed]
Prophets Dawuud and Suleiman each ruled for forty years.[citation needed]
Regarding the flood that Noah encountered, it is said that for forty days water continued to pour from the heavens and to stream out over the earth.[citation needed]
There is also a hadith from Mohammad that the prayers of a person who gossips would not be accepted for forty days and nights. (Al-Kafi, Vol. 6, p. 400)
Imam Ali has narrated from Mohammad that one who memorizes and preserves forty hadith relating to their religious needs shall be raised by God as a learned scholar on the Day of Resurrection.[citation needed]
It is said that a person’s intellect attains maturity in forty years, everyone according to his own capacity.[5]
It is believed that one who assists a blind man for forty steps becomes worthy of entering heaven.[citation needed]
Imam Baghir has said: “The prayers of someone who drinks wine are not accepted for forty days.”[citation needed]
Believers have also been encouraged to devote themselves to God Almighty for forty days to see the springs of wisdom break forth from their hearts and flow from their tongues.[citation needed]
Mourning period officially last for 40 days.
Funerary customs
Some Russians believe that ghosts of the dead linger at the site of their death for forty days.
Many Christian Filipinos mark the end of the initial mourning period on the fortieth day after death, and have a Mass said. They believe that the soul remains on the earthly plane for forty days before entering the afterlife, recalling how Christ ascended to heaven forty days after his Resurrection.
It goes on and on but these I thought were very interesting. :)