Tamriel has thirteen constellations, twelve of which can be "birthsigns". Each birthsign is assigned to a specific month, and may have a "guardian constellation".

The italicized quotes were taken from The Firmament, which is a book that details the benefits and disadvantages of each sign.

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Morning Star

• Equivalent to January
• The Ritual
  • "Those born under this sign have a variety of abilities depending on the aspects of the moons and the Divines."
  • Charge of The Mage
  • Those born under the Ritual can heal themselves faster than other signs, and are also naturally skilled in the school of Conjuration, which allows them to raise corpses in their defense for a short time.

Sun's Dawn

• Equivalent to February
• The Lover
  • "Those born under the sign of the Lover are graceful and passionate."
  • Charge of The Thief
  • Those born under the Lover are naturally gifted in magicka, stealth, and melee and learn all three disciplines a little faster but to a lesser degree than signs such as the Warrior or Mage. That being said, they are still a little more skilled in stealth-related skills than the other two. Lovers are also more agile than other signs.

First Seed

• Equivalent to March
• The Lord
  • "Those born under the sign of the Lord are stronger and healthier than those born under other signs."
  • Charge of The Warrior
  • Those born under the Lord are 25% more resistant to magical attacks and are hardier than others, able to take a lot of damage in battle.

Rain's Hand

• Equivalent to April
• The Mage
  • "Those born under the Mage have more magicka and talent for all kinds of spellcasting, but are often arrogant and absent-minded."
  • Guardian of the Ritual, the Atronach, and the Apprentice
  • All signs with the Mage as their guardian constellation also gain the benefit of natural proficiency in spellcasting.

Second Seed

• Equivalent to May
• The Shadow
  • "The Shadow grants those born under her sign the ability to hide in shadows."
  • Charge of The Thief
  • Able to turn yourself invisible at will once a day, for a limited time.
  • Argonians born under this birthsign are dubbed "Shadowscales"; they are given to the Dark Brotherhood when they are born and trained to be assassins.

Midyear

• Equivalent to June
• The Steed
  • "Those born under the sign of the Steed are impatient and always hurrying from one place to another."
  • Charge of The Warrior
  • Those born under this sign are naturally stronger and can carry more than some other people. Even the heaviest of armors is light as a feather when worn by Steeds.

Sun's Height

• Equivalent to July
• The Apprentice
  • "Those born under the sign of the apprentice have a special affinity for magick of all kinds, but are more vulnerable to magick as well."
  • Charge of The Mage
  • All those born under the Apprentice are able to regenerate their magicka faster, but also are more vulnerable to magical attacks. Inexperienced mages may be easily killed if struck by a powerful enough spell.

Last Seed

• Equivalent to August
• The Warrior
  • "Those born under the sign of the Warrior are skilled with weapons of all kinds, but prone to short tempers."
  • Guardian of the Lord, the Lady, and the Steed
  • All signs with the Warrior as their guardian constellation also gain the benefit of natural weapon proficiency.

Hearthfire

• Equivalent to September
• The Lady
  • "Those born under the sign of the Lady are kind and tolerant."
  • Charge of The Warrior
  • Those born under the Lady are naturally healthier and have more stamina, allowing them to endure more than others.

Frostfall

• Equivalent to October
• The Tower
  • "Those born under the sign of the Tower have a knack for finding gold and can open locks of all kinds."
  • Charge of The Thief
  • All those born under the Tower have a natural affinity for picking particularly difficult locks.

Sun's Dusk

• Equivalent to November
• The Atronach
  • "Those born under this sign are natural sorcerers with deep reserves of magicka, but they cannot generate magicka of their own."
  • Charge of The Mage
  • All those born under the Atronach may find it easier to cast higher-cost spells earlier on, but find it more difficult to regenerate their magicka naturally and can deplete their stores completely if not conservative. Unlike their "sibling constellation", the Apprentice, Atronachs are more resistant to magic attacks.

Evening Star

• Equivalent to December
• The Thief
  • "Those born under the sign of the Thief are not typically thieves, though they take risks more often and only rarely come to harm. They will run out of luck eventually, however, and rarely live as long as those born under other signs."
  • Guardian of the Tower, the Shadow, and the Lover.
  • All signs with the Thief as their guardian constellation also gain the benefit of natural stealth and proficiency in marksmanship. They are also more charming and quick-witted than other birthsigns.

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The Serpent

• "Those born under this sign are the most blessed and the most cursed."
• Has no specific guardian or season.
• Unlike the other signs, there are no characteristics that unite all those born under the Serpent. The Serpent wanders through the other constellations, never staying in the same place. The guardian constellations protect their charges from the Serpent.