Our family (me, my wife, son aged 14, daughter 19) were in the car driving westbound on a local secondary road at 5:30pm local time on Jan 4, 2009. The western sky was still bright although the sun had just gone over the horizon. I was sitting in the front passenger seat. Looking straight ahead, I saw a bright object come into view at the top of the Acura MDX's windshield, moving in a steady line (and at constant speed) straight down. At the rate it was moving, I knew we would have a good 7 to 8 seconds to observe it before it went below the treeline.At first I thought it could be a plane's landing light (Westchester Airport is about 20 mi west of us). It took me about 2 secs to realize it could not be a plane (because of its trajectory). At that point I said out loud: "What the heck is that??" Everyone in the car looked straight ahead and saw the object immediately (I didn't have to point it out). We watched it travel in a smooth, straight, vertical line, at constant speed, downward, until it passed below the treeline. There was no ensuing flash, sound, percussion wave, fire, smoke, or anything else. It simply disappeared from view beyond the treeline.I saw the object for about 8 seconds total. (It took this long to move from the top of the Acura windshield to the treeline.) My family saw it for about 5 to 6 seconds.The object was round and bright yellow. It was approx. 10 times too big to be Venus or a planet. It was like seeing an indoor/outdoor floodlight shining directly at you from a distance of 100 to 200 feet. It gave off light but not so much as to make you squint or look away. The light was yellowish. The object had no discernible features: just round and solid yellow. It did not change size or brightness and did not trail smoke, sparks, or debris. It was moving too slow to be a meteor (unless it was very large and far away). The motion was straight down, at a constant speed. I have no good way to estimate the distance. It had to be more than a half mile away, because otherwise we would have seen it hit the ground ahead of us. It could have been several miles away. I doubt it was more than (say) 5 or 6 miles away, because then it would had to have been extremely large and would have been seen by many, many people in the greater Stamford-Norwalk area, possibly even as far away as White Plains, NY or the Bronx.I've been a pilot for 42 years and consider myself an accurate, dispassionate observer of things airborne. What we saw was not a firework, nor (IMHO) an airplane or a piece of an airplane, nor a balloon, nor a meteor. It didn't seem bright enough to be any kind of flare (and trailed no smoke). But it did appear to emit its own light.After it disappeared from view, we were stunned. We were saying things like "Oh my God, what was that?" "What do you think that was?" When we reached our destination about 3 minutes later, we were still talking about it. We've seen no local news reports on it. It was a cold Sunday at dusk with little traffic on the streets and we feel certain very few other people saw this, unless it was further away and much bigger than we think.