• Question: Does the moon actually pull the sea towards it when its low in the sky and close to earth...

    Asked by roxannebrace to Dalya, Derek, Sarah, Tim, Tom on 20 Jun 2011.
    • Photo: Dalya Soond

      Dalya Soond answered on 17 Jun 2011:

      that’s what I have been led to believe.

    • Photo: Tim Millar

      Tim Millar answered on 17 Jun 2011:

      It pulls the ocean but its no closer to the earth when its lower in the sky. There is a famous optical illusion which make the moon look bigger when it is near the horizon though.

    • Photo: Derek McKay-Bukowski

      Derek McKay-Bukowski answered on 17 Jun 2011:

      Yes, the tides are caused by the gravitational pull of the moon (and the sun too)

      However, the moon is not necessarily closer to the earth when it is “low in the sky”. The moon’s orbit is nearly circular, but not quite. Actually, it is an elliptical orbit. (See my answer this this other question: http://ias.im/42.1217 ). This means that there are times when the moon is closer to the earth and, yes, the tides are stronger then.

      However, the moon can be closer to the earth at different times of the day and it will depend also on the time of the month (as it is also an orbital effect).

      The moon can “look” bigger, when it is near the horizon, but that is actually an optical illusion.

      Tides are also affected by the sun. And the fact that the sun and moon are at different angles and distances (even if only slightly) will affect the rhythm of the tides and when we get larger or smaller ones.

    • Photo: Tom Crick

      Tom Crick answered on 18 Jun 2011:

      The interaction of gravity in our solar system is complex, culminating in the three-body problem in classical physics: the Sun, the Earth and the Moon all have a gravitational effect on each other, which is proportional to their mass, as well as their distance from each other.

      So clearly the Sun has a strong gravitational effect on the Earth and Moon (hence the solar system!), but the Earth also exerts a gravitational pull on the Moon (hence why it is a satellite). But the Moon in turn has a gravitational effect on the Earth.

      Therefore, tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth, resulting in both short-term and long-term tidal effects across the Earth.

    • Photo: Sarah Thomas

      Sarah Thomas answered on 20 Jun 2011:

      I really like Derek’s explanation, I’d go with that 🙂

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