ASTRONOMY HELP!!!!!!!!!?

Okay, in my class, we are currently learning about Comet Holmes. Last week we learned that the comet has a semimajor axis of 3.6AU and an eccentricity of 0.43. The last time the comet was on its orbit was 2007. I have been asked to calculate the orbital period of Comet Holmes, find out the next year that the comet will be at perihelion and when will it be at aphelion, and if the comet will be coldest at perihelion or aphelion. Here are my answers...am I right?

Orbital period: P^2=A^3; So I plugged in 3.6AU to equal A, which gave me 6.831 as the orbital period

For the year pf perihelion, I just added 7 years to the 2007 date which gave me 2014...I got the 7 years by rounding up 6.831 and for the aphelion my answer was 17.854. I plugged in 6.831 fo A again. So my date would be sometime in 2025.

I believe that that the comet would be at Perhelion...place on orbit closest to the sun. Thanks!

Comments

  • Your aphelion of 17.854 is 6.831^(3/2) which is wrong. You have the period as 6.831 years, so the time from perihelion to aphelion is simply half that, i.e. 3.4155 years.

    All the rest seems O.K.

  • Where on its orbit was it in 2007? On May 4, 2007 it was at perihelion.

    If it was at perihelion then, it will be at perihelion every 6.831 years after that (your calculation is good).

    From perihelion to aphelion (the coldest spot), is half an orbit (half of 6.831 years).

    ----

    Using the Julian Day calendar (a day-count calendar), May 4, 2007 was day number 2,454,225.

    If you need to get really technical about it, the 6.831 years you found are sidereal years (one sidereal year = one full orbit of Earth relative to the fixed stars) of 365.259635 days

    (we'll use 365.26)

    365.26*6.831 = 2330.72

    The next (calculated) perihelion will be on day number:

    2,454,225 + 2331 = 2,456,556 = 20 September 2013 (or September 19, depending on your time zone).

    If you divide 2,456,556 by 7, you get a remainder of 4, which means a Friday (In the Julian Day calendar, all Mondays have a remainder of 0).

    The time from perihelion to aphelion is only half an orbit:

    2330.7 / 2 = 1165.35 (let's use 1165)

    Date of Aphelion = 2,454,225 + 1165 = 2,455,390 = July 12, 2010 (or 13th, depending on your time zone).

    2,455,390 is divisible by 7 (it leaves a remainder of 0), therefore it is a Monday.

    ---

    This is a good way to learn about Kepler's law (a^3 = P^2). In reality, astronomers will also work in other factors (for example, they will estimate the effect of the giant planets on the comet's orbit, and any effect of the outgassing which often slows down the comet a tiny bit on its way in and accelerates it a tiny bit in the early parts of its way out.

    Comet Holmes gave off so much gas in October 2007 that it temporarily became bigger than the Sun (of course, the difference was that it was a very tenuous gas with almost no density -- if a spacecraft had entered this 'sphere' of gas, it would not have felt anything and it would not even have slowed down).

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