日晷又称为日规，是一种由太阳位置告知当天时间的设备。在设计上，有一个晷针投射出阴影在刻度盘上，经由盘上的标线指示出时间。在西方最常见的是水平日晷，在中国则是赤道日晷。晷针提示时间的方式通常是以细杆、明锐边缘的投影，当太阳移动着划过天际，阴影边缘会与不同的时间线对齐。所有形式的日晷都必须对正方位，其晷针必须平行于地球的自转轴，才能在整年都提供正确的时间。因此晷针与水平面的夹角必须等于所在地的地理纬度，而一般常见廉价的装饰日晷是大批量产的，所以晷针的角度与时角是不正确的，也就不能提供正确的时间。

A sundial is a device that tells the time of day by the apparent position of the Sun in the sky. In the narrowest sense of the word it consists of a flat plate (the dial) and a gnomon which casts a shadow onto the dial. As the sun appears to move across the sky, the shadow aligns with different hour-lines which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, a wire or an elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude.