The problem at the time was that there was no accurate way of determining the time at two different places at once. Pendulum clocks had already been invented and used, but on the deck of a rolling ship, such clocks would slow down, or speed up, or stop running altogether. Warm and cold temperatures thinned or thickened a clock's lubricating oil and made its metal parts expand or contract, thus slowing or speeding up tremendously. A rise or fall in barometric pressure, as well as variations in Earth's gravity from one latitude to another, caused a clock to gain or lose time. These watches typically gained or lost as many as fifteen minutes a day. Since one degree of longitude equals four minutes of time, fifteen minutes a day would equal to almost four degrees of longitude. Near the equator, that would translate to almost two hundred and seventy-two miles. In only one day of navigation, the ship would be two hundred and seventy-two miles off course! Imagine a trip across the Atlantic of many months.