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# Physics/Year

Question
Hello,

How can a year of time be both Sidreal and Lunar ?

Which one is exactly a 360 degree orbit ?

Thanks,
Serena

Please forgive my delay in responding -- it's the only way I can think of, to ensure I am not assisting with academic work, of which homework is just a small part. Also, as I am unable to determine the veracity of what people post, I can not know whether or not a question involves academic work.

Like any unit of measure, a year can be anything people choose to define it to be. A "troy" ounce, for example, is different from a "avoirdupois" ounce; simply because various people have decided to define an "ounce" in different ways.

A "lunar year" is defined as twelve complete cycle of our Moon; ie, twelve "months." Note, however, that one complete moon cycle, as viewed from our Earth, is about 29.5 days. Thus, one lunar year is about 354 days -- ten days shorter than a complete revolution of our Earth around our Sun. Some societies simply ignore the discrepancy, and have a lunar calendar in which a specific month could occur during any season (spring, summer, winter). Other societies reset their lunar calendars so that a specific month will always occur in a specific season.

A "sidereal year" is the length of time it takes for our Earth to make a full rotation around our Sun, as viewed from the distant stars ("sidereal" = "of the stars"). This is the one that means a 360 degree orbit.

Permit me to add the phrase, "tropical year," which is the time between identical solar equinoxes, as viewed from one point on our Earth's surface. This measurement of a "year" is very slightly shorter than a sidereal year. The difference is due to the fact that the direction of the rotation axis of our Earth is precessing VERY slowly in relation to the stars. Thus, an observer sitting at one spot on our Earth will observe the time between one equinox and the next as just a little less time than a sidereal year.

Physics

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I can help with understanding physics that does not involve eggs. I will NOT help with academic or professional questions, which are NOT limited only to homework. Please do not waste your time by asking a question that comes out of ANY kind of academic, professional, or business matters.

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Have been fascinated by physical laws ever since I learned, at age seven, that magnets work under water. My study continued through college and has not ceased even after I retired.

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B.A. in Physics (with honors) from University of California at Berkeley.M.A. in Physics (with honors) from University of Texas Austin.