What is a sidereal month and why is it dfferent than a synodic month?

Answer 1

Synodic month is longer than Sidereal month because when the moon completes 1 revolution round the earth, it will not be a full moon as the earth also moves in its orbit

Answer 2

Synodic comes from the Latin word synod meaning "meeting" so the moon has to "meet" with the sun every new moon. A synodic month takes 29 1/2 days

A Sidereal month takes only about 27 & 1/3 days, THE MOON MUST COMPLETE MORE THAN ONE FULL ORBIT OF EARTH FROM ONE NEW MOON TO THE NEXT.

that's why a sidereal & synodic month are not the same length

Answer 3

The moon orbits eastward from the earth in 27.32 days. this is called the sidereal period because it refers to a fixed stars. Although the moon orbits earth in 27.32 days, it takes slightly longer to go through its phases. While the moon moves around the sky in 27.32 days, the sun moves eastward about 1 degree per day. In 27.32 days, the moon returns to the same place in the sky where it was last seen, but the sun has moved about 27 degree east, so the moon needs slightly more than 2 days to catch up with the sun and reach new moon again. Thus, one cycle of lunar phases takes 29.53 days an interval called the lunar synodic (rather than "synoptic") period.

Answer 4

It has to do with the facts that 1) a synodic month is calculated based on the moon's position from our point of view relative to the sun, and 2) both the orbit of the moon around the earth and the orbit of the earth around the sun are anticlockwise, and 3) the sidereal month views the position of the moon and earth relative to the fixed stars, ignoring the moon's position relative to the sun.

The above may be difficult to visualize, but with a little thought it becomes clear. Imagine sticking a strip of cardboard to the wall with a tack, or actually do it if the owner of the wall is ok with it. Turn the strip toward the right when it is pointing down and keep going, then to the left when it is pointing up; that is anticlockwise. Imagine that the earth located where the strip is tacked to the wall, and the moon is tacked to the end of the strip. Both the earth and the moon are free to spin on the strip in the anticlockwise direction.

If you imagine the strip pointing straight out to the right, the 3 o'clock position, that is our starting point. Now turn the strip anticlockwise exactly one complete turn, and that is a sidereal month. For sidereal months you ignore what is happening relative to the sun, and that is what you are doing. You can keep turning the strip exactly one whole turn anticlockwise, over and over again, and you will continue to trace out sidereal months.

But position relative to the sun is how you trace out synodic months, and these are the months that are in sync with the moon phases that we observe. That makes perfect sense, because the phases of the moon are determined by how much of the moon's surface is illuminated by the sun.

So turning the strip exactly one time anticlockwise doesn't show what is happening for observers on earth. During the period of time that the moon orbits anticlockwise, the earth is also moving anticlockwise around the sun. To go precisely from one full moon to the next full moon, you have to spin the strip one full turn anticlockwise and then a little extra, to account for this movement of the earth. Hence, we have synodic months that are a little longer than sidereal months.

Answer 5

That is true for most planets, but it actually depends on the direction of the planet's rotation. In fact, on Venus the sidereal day is longer than the solar day.

That's because of the "retrograde" rotation of Venus.

The "sidereal day" is the rotation period of a planet relative to the stars.

That's the "real "rotation period.

The "solar day" is the rotation period relative to the Sun's position in the sky.

Take the Earth as an example. Earth orbits the Sun as it rotates on its axis.

The Earth needs to rotate a little more than one sidereal day to compensate for the change in the Sun's position in our sky that is caused by the Earth's orbital movement around the Sun.

The Earth requires about 4 minutes of extra rotation to put the Sun back in the

"correct" place in the sky. Earth's solar day of 24 hours is about 4 minutes longer than its rotation period (the sidereal day).

Answer 6

The "month" in this case refers to how long it takes for the Moon to go once around Earth. Depending on the reference point chosen, different lengths of months are possible. Sidereal means that the background stars are used as a reference point; synodic means that the Sun is chosen as a reference point (for example, from one new moon to the next). Since Earth moves around the Sun, the Sun's angle among the star changes, so the two reference points are not equivalent.

Answer 7

A sidereal month is the time it takes the moon to complete one orbit of earth. A synodic month is longer than a sidereal month because it takes longer for the moon to go through it's phases