The two planets that are not seen in the sky at night are Mercury and Venus. This is because they are inner planets, meaning they orbit closer to the Sun than Earth does.

Here's why:

* They are close to the Sun: Inner planets are always close to the Sun in the sky.

* Short Orbital Period: Mercury and Venus orbit the Sun faster than Earth. This means they spend most of their time close to the Sun, either in the morning sky (before sunrise) or the evening sky (after sunset).

* Limited Visibility: When Mercury and Venus are near the Sun, they are either lost in the glare of the Sun or are only visible for a short time right before sunrise or after sunset.

Therefore, while you might occasionally catch a glimpse of Mercury or Venus in the twilight sky, they are rarely visible at night like the outer planets (Mars, Jupiter, Saturn, Uranus, and Neptune).