If the Earth did not have a moon, we would still have day and night because day and night are caused by the Earth's rotation on its own axis, not by the moon. The Earth rotates approximately once every 24 hours, causing different parts of the planet to face the Sun or face away from it, which creates the cycle of daytime and nighttime. However, without the moon, some related effects would change dramatically:

1. Earth's rotation would likely be faster, resulting in shorter days, possibly only 6 to 12 hours long initially, because the moon's gravitational pull currently slows Earth's spin over time.
2. The moon stabilizes Earth's axial tilt (about 23.5 degrees). Without it, Earth's axis could wobble chaotically, leading to unpredictable and extreme changes in the length of day and night over long timescales as well as wild variations in seasons.
3. Tidal effects would be weaker because tides caused by the moon would disappear, leaving only the sun's weaker tides, but this does not affect day and night.

Thus, day and night would still exist without the moon, but the length of a day would be shorter, and the stability of Earth's tilt, which influences the regularity of days and seasons, would be compromised.