public string GetAtomFromBirthday(DateTime birthday)
{
    float birthdayF = 0.00F;

    if (birthday.Month == 1 && birthday.Day < 20)
    {
        birthdayF = float.Parse(string.Format("13.{0}", birthday.Day));
    }
    else
    {
        birthdayF = float.Parse(string.Format("{0}.{1}", birthday.Month, birthday.Day));
    }
    float[] atomBound ={ 1.20F, 2.20F, 3.21F, 4.21F, 5.21F, 6.22F, 7.23F, 8.23F, 9.23F, 10.23F, 11.21F, 12.22F, 13.20F };
    string[] atoms = { "水瓶座", "雙魚座", "白羊座", "金牛座", "雙子座", "巨蟹座", "獅子座", "處女座", "天秤座", "天蠍座", "射手座", "魔羯座" };

    string ret = "靠！外星人啊。";
    for (int i = 0; i < atomBound.Length - 1; i++)
    {
        if (atomBound[i] <= birthdayF && atomBound[i + 1] > birthdayF)
        {
            ret = atoms[i];
            break;
        }
    }
    return ret;
}