perihelion

Comet C/2012 S1 ISON at Perihelion

Today comet C/2012 S1 ISON will pass through perihelion, its closest approach to the Sun.

At a mere 1,800,000km (1,100,000miles) this will be a close pass indeed. As perihelion is measured from center to center, the distance is even closer if you consider the 695,500km (432,200mile) radius of the Sun. Subtracting the solar radius you realize the comet will pass a mere 1,100,000km (680,000miles) above the surface of the Sun. At this distance the intensity of the solar radiation will be nineteen thousand times more intense than a sunny day on Earth. Hot indeed!!

C/2006 P1McNaught — C/2006 P1 McNaught while 5° from the Sun on Jan 14,2007

While the comet is so close to the Sun volatile gasses will be streaming off the comet in huge quantities creating an extremely bright comet. It is fairly likely that the comet will be visible in the daytime. If so, it will appear much as C/2006 P1 McNaught appeared in January 2007 just a few degrees from the Sun, potentially visible to the unaided eye.

At closest approach the comet will be less than a degree from the Sun, difficult to pick out. An observers best bet will be as it approaches and as it moves away from the solar disk. As the comet nears perihelion it will approach the Sun from the west, best seen in the dawn sky. After perihelion it will exit the Sun’s vicinity to the north, favoring northern hemisphere observers.

The comet should be spectacular in the cameras of the dedicated solar observation satellites. Check out the real time views from SOHO or Stereo.

Observing ISON through Perihelion

Plotted below is the path of comet ISON through perihelion. The image is zenith up on the morning of November 28th from the island of Hawai’i. The actual moment of perihelion will be Nov 28.77501UT (18:36UT or 08:36HST).

A few things are notable… The obvious one is how close the comet will get to the Sun. Not just in absolute terms, which is really close. But rather how close it will look to us. The comet will be under 30 arc-minutes from the center of the Sun, recalling that the Sun is about 30 arc-minutes across. The comet will not pass behind the Sun from our point of view. While we may not be able to see it while lost in the solar glare, it will remain in the view of those solar monitoring spacecraft that are near the Earth.

Separation will help in trying to spot the comet during the day. During the 27th, 28th and 29th the comet will be very close to the Sun. On the morning of the 29th the comet will be only 4.5° from the Sun. Best bet to attempt a daytime peek may be on the 30th or later, when the comet will again be more than 7.5° from the Sun. Look for the magnitude estimates and be prepared to give it a look.

ISON at Perihelion — The orbit of comet C/2012 S1 ISON as it passes through perihelion on November 28th, 2013

Comet C/2011 L4 Pan-STARRS at Perihelion

Today Comet C/2011 L4 (Pan-STARRS) will pass through perihelion, its closest approach to the Sun. Take a good look now, given the 110,000 year orbital period, the comet will not be back anytime soon.

The comet should be visible as a 1st magnitude object 13° above the horizon at sunset and directly above the glow of the setting Sun. It will climb higher each evening after passing through perihelion. It will begin to fade as well, most likely dimming below 4th magnitude by the end of March.

Today the Earth is closest to the Sun, a point called perihelion. We will be about 147,099,000km (91,403,000miles) from the Sun. Compare this to the 152,096,000km (94,508,000miles) we will be at aphelion on July 4th, a difference of about 4,996,000km (3,104,000miles) occurs throughout one orbit.

It may seem odd that we are actually at the closest for the middle of northern winter, you just have to remember that proximity to the Sun is not the cause of the seasons. The seasons are caused by the axial tilt of the Earth, creating short and long days throughout the year, with a resulting change in the angle and intensity of the sunlight.

2013 Solstices and Equinoxes
	UT		HST
Perihelion	Jan 2	00:59UT	Jan 1	14:59HST
Spring Equinox	Mar 20	11:02UT	Mar 20	01:02HST
Summer Solstice	Jun 21	05:04UT	Jun 20	19:04HST
Apehelion	Jul 5	18:59UT	Jul 5	08:59HST
Fall Equinox	Sep 22	20:44UT	Sep 22	10:44HST
Winter Solstice	Dec 21	17:11UT	Dec 21	07:11HST

Source: NASA Sky Calendar

Perihelion

Today the Earth is closest to the Sun, a point called perihelion. We will be about 147,098,290km (91,402,639miles) from the Sun. Compare this to the 152,098,232km (94,509,459miles) we will be at aphelion on July 4th, a difference of about 5,411,169km (3,362,344miles) occurs throughout one orbit.

2012 Solstices and Equinoxes
	UT		HST
Perihelion	Jan 5	03:59UT	Jan 4	17:59HST
Spring Equinox	Mar 20	05:14UT	Mar 19	19:14HST
Summer Solstice	Jun 20	23:09UT	Jun 20	13:09HST
Apehelion	Jul 4	23:59UT	Jul 4	13:59HST
Fall Equinox	Sep 22	14:49UT	Sep 22	04:49HST
Winter Solstice	Dec 21	11:12UT	Dec 21	01:12HST

Source: USNO Data Services Website and the NASA Sky Calendar