Why can we Always see the Planets?!

[wilddog]

Hey guys, my first post here. Just have a general question that I have been racking my brains over & just cant figure out.<br /><br />Why can we always see planets (Jupiter for example) on any given night in the year??<br />I figured that if we are completing our orbit in 1 year & Jupiter takes approximately 12 years, at some point, Earth & Jupiter are going to be on opposite sides of the Solar system with the sun inbetween......Clearly, if this was the case then even with a telescope we probably would not be able to see the planet....But every night throughout the year(s), we can see Jupiter!!<br />WHY???<br />Cheers & thanks in advance,<br />WD

 

[doubletruncation]

Hi and welcome to SDC!<br /><br />You're right that Jupiter and Earth will occasionally be on opposite sides of the sun from each other, on that day Jupiter will rise at almost the same time the sun does and set at almost the same time the sun does - so in fact you wouldn't really be able to see Jupiter at night. If Jupiter is slightly to the East of the sun in the sky then it will set just after the sun sets, but will be up at night for that portion of time between when the sun sets and it sets. If jupiter is slightly to the west of the sun in the sky then it will rise just before the sun rises, but will be up at night for that portion of time between when it rises and the sun rises. In that sense it will be up at some point on almost every night of the year. (I've been assuming here that you're at a latitude where the sun and jupiter rises above the horizon every day). <div class="Discussion_UserSignature"> </div>

 

[wilddog]

Thanks so much for your reply mate, it really helped!!<br /><br />But one thing is still not clear....According to my calculations, every six years, (approx.) Earth & Jupiter will be on opposite sides of the solar system & not for just 1 night.....for a series of nights......& I was always under the impression (I am probably wrong) that Jupiter is always visible to the naked eye on every single night of the year.<br />Suppose instead of Jupiter, we take Venus. Venus is definately vissible to the naked eye on every single night of the year, regardless of whether its on the other side of the solar system!!<br />Also, does the rule that you stated above apply to all the planets??<br />Cheers & thanks so much for any help. This site is an amazing resource....Was just browsing & there are some really amazing threads!!

 

[ughbug]

Not just every six years, every year. Remember, 1 earth year equal one rotation around the sun. This means that the earth will be on the "opposite" side of the solar system of every planet every year. During that time, the planet disappears from view.<br /><br />Matt

 

[why06]

Well ofcourse all the planets aren't visible all of the time, but most are most of the time. I live in tennessee. Though we see the morning star. I can usually see it the best during fall. I'm pretty sure that doesn't mean Its not there most of the time but, that is the time I see it the most...just before we turn the clocks back.<br /><br /><br />..hmmm does that mean New Years is always one hour off every year? <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <div>________________________________________ <br /></div><div><ul><li><font color="#008000"><em>your move...</em></font></li></ul></div> </div>

 

[wilddog]

But cant we see Venus every night throughout the year???

 

[CalliArcale]

No. When it gets very close to the Sun (as seen from Earth's perspective), Venus cannot be seen with the naked eye. This is called "inferior conjunction".<br /><br />There are two rare circumstances that can make Venus visible during inferior conjunction: a total solar eclipse, or a Venus transit. The latter is far more rare, but one's coming up in 2012. That'll be the last one in our lifetimes. (Venus transits come in pairs; the first one of this pair occurred in 2004. And it was cloudy here in Minnesota, dammit.) <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[MeteorWayne]

Welcome to SDC. Great question.<br />I see you've gotten some good answers, let me add my comments.<br /><br />Mercury and Venus are a special case, since they are closer to the sun than we are. So when and where they are visible is more determined by their orbit.<br />Mercury's orbit is about 88 days, so every 44 days it is either between our location at one time and the sun, or behind the sun. (That's called inferior and superior conjunction, BTW). Of course , since we are orbiting, too, the actual interval is about 115 days between inferior conjunctions. In between, about 29 days after each it is at it's greatest distance away from the sun, and is visible in the morning or evening sky. Since mercury's orbit is so small, it appears only during twilight.<br />For Venus' orbit of about 225 days, and with our orbit included, the interval between conjunctions is about 293 days, and 146 or so days later it's farthest from the sun on either side. Since it's orbit is larger, the sky can be dark when it's at that spot.<br /><br />For all the other planets, their visibility is controlled mostly by our orbit as the piece of the sky that the sun hides changes throughout the year. During the year, the planet will be in conjunction (behind the sun), about 6 months later it will be at the opposite point in the sky(called opposition) and be visible all night long. The exact timing changes, since all those planets are in their own orbits as well. That's most noticeable for mars, which makes one orbit for a little less than any two of ours. For all the others, the year to year change is much smaller. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[MeteorWayne]

That changes from year to year, over a period of decades the morning and evening appearences of Venus will occur at all times of the year. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[wilddog]

All points taken!!!<br />Cheers & thanks a million fellas!!

 

[pizzaguy]

WATCH IT! Calli isn't a "fella". <img src="/images/icons/crazy.gif" /> <div class="Discussion_UserSignature"> <font size="1"><em>Note to Dr. Henry:  The testosterone shots are working!</em></font> </div>

 

[centsworth_II]

Isn't fella the feminine of fellow? <div class="Discussion_UserSignature"> </div>