month

• Russian
  ме́сяцmesjac

  ˈmʲesʲɪtsinherited

  Also the crescent moon.

• Ukrainian
  мі́сяцьmisjacʹ

  ˈmʲisʲatsʲinherited

  Also the moon.

• Polish
  miesiąc

  ˈmjɛɕɔntsinherited

  Only the month; the moon is księżyc.

• Czech
  měsíc

  ˈmɲɛsiːtsinherited

  Also the moon.

• Bosnian-Croatian-Serbian
  mjesec / mesecмјесец / месец

  ˈmjesetsinherited

  Also the moon.

• English
  month

  Distant English relative: moon — Month, moon and *měsęcь share one Indo-European word for the moon.

Sound laws at work

• Jat (ě) and its reflexes
• Nasal vowels (ę, ǫ)
• Ukrainian і (ikavism)