The Chinese Zodiac initially originated from the division of time. Nowadays, we divide a day and night into 24 hours while ancient astronomers divide it into 12 shichen, which is a two-hour period, namely, zi, chou, yin, mao, chen, si, wu, wei, shen, you, xu and hai. In astronomical observation, they choose 12 animal signs corresponding to each shichen according to the living habits and the time of activity of the animals.
夜间十一点至次日凌晨一点，属子时，正是老鼠趁夜深人静，频繁活动之时，故称“子鼠”。 From 11 pm to 1 am of the next day, it is the time of “zi”. It is the time for rats to run around while people fall asleep, so we have zi, rat.
凌晨一点至三点，属丑时。牛习惯夜间吃草，农家常在深夜起来挑灯喂牛，故称“丑牛”。 From 1 am to 3 am, it is the time of “chou”. The ox is used to eating grass at night and farmers often get up in the midnight and feed them, so we have chou, ox.
凌晨三点至五点，属寅时。此时昼伏夜行的老虎最凶猛，古人常会在此时听到虎啸声，故称“寅虎”。 From 3 am to 5 am, it is the time of “yin”. Because the tiger often moves about at night and ancient people often hear them roaring at this time, we call it yin, tiger.
清晨五点至七点，属卯时。天刚亮，兔子出窝，喜欢吃带有晨露的青草，故为“卯兔”。 From 5 am to 7am, it is the time of “mao”. The hare goes out of its nest and eats green grass with morning dew on it, so we call it mao, hare.
早晨七点至九点，属辰时。此时一般容易起雾，传说龙喜腾云驾雾，又值旭日东升，蒸蒸日上，故称“辰龙”。 From 7 am to 9 am, it is the time of “chen“. It often mists at this time and legend says that dragons like to ride clouds and mist. It's also the time for the sun to rise, so we have chen, dragon.
上午九点至十一时，属巳时。大雾散去，艳阳高照，蛇类出洞觅食，故作“巳蛇“。 It is the time of “si“ from 9 am to 11 am. The mist has vanished and the sun has risen up in the sky. Snakes will go out to find food, so we call it si, snake.
中午十一点至一点，属午时。古时野马未被人类驯服，每当午时，四处奔跑嘶鸣，故称“午马”。 It is the time of “wu” from 11 am to 1 pm. Before the wild horse was domesticated in the ancient time, they usually ran around and neighed at this time. That's why we call wu, horse.
午后一点至三点，属未时。有的地方管此时为“羊出坡”，意思是放羊的好时候，故称“未羊”。 From 1 pm to 3 pm, it is the time of “wei”. In some places, people call this period “sheep on the slope” because it's the right time for herding sheep. That's why we call it wei, sheep.
下午三点至五点，属申时。太阳偏西了，猴子喜在此时啼叫，故为“申猴”。 It's the time of “shen” from 3 pm to 5 pm. The sun has moved to the western half of the sky. Monkeys love to screech at this time, so we call it shen, monkey.
下午五点至七点，属酉时。太阳落山了，鸡在窝前打转，故称“酉鸡”。 It's the time of “you” from 5 pm to 7 pm. The sun has set. Roosters go back to their nest, so we call it you, rooster.
傍晚七点至九点，属戌时。人劳碌一天，闩门准备休息了。狗卧门前守护，一有动静，就汪汪大叫，故为“戌狗”。 From 7 pm to 9 pm, it's the time of “xu”. After a busy day, people close their gates and go to sleep. Dogs guard the gates and will bark in case of any noise, so we call it xu, dog.
夜间九点至十一点，属亥时。夜深人静，能听见猪拱槽的声音，于是称作“亥猪”。 It's the time of “hai” from 9 pm to 11 pm. In the still of night, we can hear pigs dig the trough with their snouts, so we call it hai, pig.
就这样，一天的时辰和动物搭配就排列了下来：子鼠、丑牛、寅虎、卯兔、辰龙、巳蛇、午马、未羊、申猴、酉鸡、戌狗、亥猪。后来人们把这种计时法用于纪年，就出现了十二生肖。 In this way, the 12 shichen and animals are matched and sequenced: zi rat, chou ox, yin tiger, mao hare, chen dragon, si snake, wu horse, wei sheep, shen monkey, you rooster, xu dog and hai pig. Later, people use this method of time labeling in counting years. This is why we have the Chinese Zodiac.